Identity of small fairy (4)
I believe the answer is:
self
'identity' is the definition.
(I've seen this before)
'small fairy' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fairy' becomes 'elf' (elf is a kind of fairy).
's'+'elf'='SELF'
'of' acts as a link.
(Other definitions for self that I've seen before include "One's individuality" , "Will the writer?" , "Will the author" , "Personality" , "one's identity" .)