Mutant terrapins come to light (9)
I believe the answer is:
transpire
'come to light' is the definition.
(synonyms)
'mutant terrapins' is the wordplay.
'mutant' indicates anagramming the letters.
'terrapins' with letters rearranged gives 'TRANSPIRE'.
(Other definitions for transpire that I've seen before include "Prove to be so, happen" , "Turn out" , "Terrapins (anag)" , "exhale" , "Come to pass" .)