Go round old gallery after opening of retrospective (6)
I believe the answer is:
rotate
'go round' is the definition.
(thesaurus)
'old gallery after opening of retrospective' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'gallery' becomes 'tate' (I've seen this before).
'after' means one lot of letters go next to another.
'opening of' suggests taking the first letters.
The first letter of 'retrospective' is 'r'.
'o'+'tate'='otate'
'otate' put after 'r' is 'ROTATE'.
(Other definitions for rotate that I've seen before include "Turn around on axis or central point" , "wheel" , "Alternate" , "'Er, to a T? (anag)" , "Turn around an axle, swivel" .)