Large rock overturned (3)
I believe the answer is:
big
'large' is the definition.
(synonyms)
'rock overturned' is the wordplay.
'rock' becomes 'gib' (both are nicknames for Gibraltar).
'overturned' shows that the letters should be reversed in order.
'gib' back-to-front is 'BIG'.
(Other definitions for big that I've seen before include "London bell" , "Huge" , "Important" , "Massive" , "Sizeable" .)