Gone off with one in the gallery to manage (9)
I believe the answer is:
negotiate
'to manage' is the definition.
(thesaurus)
'gone off with one in the gallery' is the wordplay.
'off' is an anagram indicator.
'with' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'in the' means one lot of letters goes inside another.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'i' going inside 'tate' is 'tiate'.
'gone' is an anagram of 'nego'.
'nego'+'tiate'='NEGOTIATE'
(Other definitions for negotiate that I've seen before include "Discuss the terms of an arrangement" , "Find a way over (a difficult path)" , "treat! Yes!" , "I get a note (anag) - bargain" , "as diplomat does" .)