Think deeply in business with soldier at the gallery (8)
I believe the answer is:
cogitate
'think deeply' is the definition.
(I've seen this before)
'business with soldier at the gallery' is the wordplay.
'business' becomes 'co' (short for company).
'with' says to put letters next to each other.
'soldier' becomes 'GI' (GI can mean a US soldier).
'at' is a charade indicator (letters next to each other).
'the gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'co'+'gi'+'tate'='COGITATE'
'in' is the link.
(Other definitions for cogitate that I've seen before include "Consider" , "Ruminate" , "Meditate - reflect" , "Think deeply and long" , "Deliberate" .)