It was the White Rabbit, trotting slowly back again, looking anxiously about, as if he had lost something?
It: pronoun, dummy subject
was: linking verb
the White Rabbit: predicate nominative and true subject [I assume
"White Rabbit" is capitalized because it is a specific
entity, such as a character in a book.
trotting slowly back again: present participle + adverb + adverb + adverb
The participial phrase is adjectival, moderating "White
looking anxiously about: present participle + adverb +adverb
The participial phrase is also adjectival, modifying "White
as if: conjunction connecting the subordinate clause to the verbal "looking".
he had lost something: pronoun (subject) + had lost (verb) + something
(direct object). This is a subordinate clause.
It is, as you said, a cleft sentence. I am not familiar with the SVCA system or how it is used for complex senetences such as this one.