Go round old London gallery next to river (6)
I believe the answer is:
rotate
'go round' is the definition.
(thesaurus)
'old london gallery next to river' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'london gallery' becomes 'tate' (I've seen this before).
'next to' says to put letters next to each other.
'river' becomes 'r'.
'o'+'tate'='otate'
'otate' put after 'r' is 'ROTATE'.
(Other definitions for rotate that I've seen before include "Spin round" , "Whisk round" , "Go round and round" , "Turn round" , "Turn around as on axle or spit" .)