Massive rock turns up (3)
I believe the answer is:
big
'massive' is the definition.
('big' can be similar in meaning to 'massive')
'rock turns up' is the wordplay.
'rock' becomes 'gib' (both are nicknames for Gibraltar).
'turns up' is a reversal indicator.
'gib' written backwards gives 'BIG'.
(Other definitions for big that I've seen before include "Circus tent" , "Large; generous" , "Large or substantial" , "Massive" , "Sizeable" .)