It should have been "safely", but you will hear "safe" used in this way in colloquial English.
The following scenario is from House of Cards, which is an American TV drama.
A female journalist visited a congressman for trying to get an interview in the late night. She intentionally wore push up bra and low cut V-neck. Later the congressman walked her out to the door, when his wife returned. She told the journalist, " Drive safe, there's a lot of ice on the road".
I was told a verb should be modified by an adverb. I don't understand why in above context the woman said, "drive safe" instead of drive safely". Could you illuminate it?
Some dictionaries list 'safe' as an adverb.
Informal. in a safe manner; safely: Learn how to drive safe.
It applies particularly in the set phrase 'play safe'.
'I hope we have enough spending money for our day out. Let's play safe and take an extra £20'.
Since "play safe" is a set phrase, it is wrong to say "play safely", isn't it?
Could you confirm this?
In so far as "play (it) safe" is a set phrase I think that, by definition, if you change it it is wrong.
Granted, it's not wrong grammatically but I wouldn't expect to hear it from a native and would correct it for someone trying to speak natural English.
I'm not even sure that "play safe" is ungrammatical. What about "play it cool" or "play dead"? "Safe" could easily be an adjective with "play" having the meaning be/stay/remain. These would all be correctly followed be "safe" and not "safely".