Mock turtle's first relation (5)
I believe the answer is:
taunt
'mock' is the definition.
(taunting is a kind of mocking)
'turtle's first relation' is the wordplay.
'turtle's first' becomes 't' (ast letter of 'turtle').
'relation' becomes 'aunt' (aunt is a kind of relation).
't'+'aunt'='TAUNT'
(Other definitions for taunt that I've seen before include "Sarcastic challenge" , "Thing said to anger or wound" , "Twit" , "Rag, goad" , "Insult" .)