Polish soldiers, excellent chaps beginning to train (10)
I believe the answer is:
refinement
'polish' is the definition.
(synonyms)
'soldiers excellent chaps beginning to train' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'excellent' becomes 'fine' (synonyms).
'chaps' becomes 'men' (I've seen this before).
'beginning to' suggests taking the first letters.
The first letter of 'train' is 't'.
're'+'fine'+'men'+'t'='REFINEMENT'
(Other definitions for refinement that I've seen before include "Elegance, sophistication" , "Improvement; elegance" , "culture" , "Cultivation" , "Polish" .)