Playing gig in ACT? Tremendous! (8)
I believe the answer is:
gigantic
'tremendous' is the definition.
(synonyms)
'playing gig in act?' is the wordplay.
'playing' indicates an anagram.
'gig'+'in'+'act'='giginact'
'giginact' is an anagram of 'GIGANTIC'.
(Other definitions for gigantic that I've seen before include "Exceedingly large" , "Great big" , "'Enormous, mammoth (8)'" , "walloping" , "Sizeable" .)