Soldiers shut up in the bog get spruced up? (7)
I believe the answer is:
freshen
'get spruced up?' is the definition.
'soldiers shut up in the bog' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'shut up' becomes 'sh'.
'in' indicates putting letters inside.
'the bog' becomes 'fen'.
're'+'sh'='resh'
'resh' placed into 'fen' is 'FRESHEN'.
(Other definitions for freshen that I've seen before include "Grow stronger (wind)" , "(Wind) get stronger" , "(Of wind) become stronger and colder" , "Make less stale" , "Re-invigorate" .)