A small bird, an old giant (5)
I believe the answer is:
'old giant' is the definition.
(I've seen this before)
'a small bird an' is the wordplay.
'a small bird' becomes 'tit' (I've seen this in another clue).
(Other definitions for titan that I've seen before include "Goddess" , "One of the first pantheon of Greek deities" , "Mythological giant" , "Atlas, possibly" , "bigwig" .)