Rock's returned, being oversized (3)
I believe the answer is:
big
'oversized' is the definition.
('big' can be similar in meaning to 'oversized')
'rock's returned' is the wordplay.
'rock' becomes 'gib' (both are nicknames for Gibraltar).
'returned' is a reversal indicator.
'gib' reversed gives 'BIG'.
'being' is the link.
(Other definitions for big that I've seen before include "Huge" , "Of considerable size" , "Circus tent" , "Large; generous" , "Massive" .)