Soldiers getting punished to become improved (7)
I believe the answer is:
refined
'improved' is the definition.
(refining is a kind of improving)
'soldiers getting punished' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'getting' says to put letters next to each other.
'punished' becomes 'fined' ('fined' can be similar in meaning to 'punished').
're'+'fined'='REFINED'
'to become' acts as a link.
(Other definitions for refined that I've seen before include "Just like the cultured" , "Not coarse" , "With impurities removed" , "Freed from impurities by processing" , "Elegant, cultured" .)