Grand eel, when wriggling, became bigger (8)
I believe the answer is:
enlarged
'became bigger' is the definition.
(enlarging is a kind of becoming bigger)
'grand eel when wriggling' is the wordplay.
'when wriggling' indicates an anagram.
'grand'+'eel'='grandeel'
'grandeel' is an anagram of 'ENLARGED'.
(Other definitions for enlarged that I've seen before include "Increased, augmented" , "Elaborated or made bigger" , "Magnified" , "Bloated" , "Made bigger, say photo" .)