Gallery object is a welcome surprise (7)
I believe the answer is:
godsend
'a welcome surprise' is the definition.
(I've seen this before)
'gallery object' is the wordplay.
'gallery' becomes 'gods' (I've seen this before).
'object' becomes 'end' (I've seen this in other clues**).
'gods'+'end'='GODSEND'
'is' acts as a link.
(Other definitions for godsend that I've seen before include "Stroke of luck" , "Unexpected help" , "Very opportune thing" , "A piece of unexpected good fortune - despatched from above?" , "Boon - blessing" .)