Fluster a soldier next to gallery (7)
I believe the answer is:
agitate
'fluster' is the definition.
(I've seen this before)
'a soldier next to gallery' is the wordplay.
'soldier' becomes 'GI' (GI can mean a US soldier).
'next to' says to put letters next to each other.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'a'+'gi'+'tate'='AGITATE'
(Other definitions for agitate that I've seen before include "Perturb" , "Campaign" , "Disturb, cause to worry" , "Upset, disturb the surface of" , "Stir violently" .)