Mock turtle's original relative (5)
I believe the answer is:
taunt
'mock' is the definition.
(taunting is a kind of mocking)
'turtle's original relative' is the wordplay.
'original' suggests taking the first letters.
'relative' becomes 'aunt' (aunt is a kind of relative**).
The initial letter of 'turtles' is 't'.
't'+'aunt'='TAUNT'
(Other definitions for taunt that I've seen before include "Tease hurtfully" , "Provoke; tease" , "element of teasing" , "Mock provokingly" , "Goad, provoke" .)