Detachment of soldiers to do military work (7)
I believe the answer is:
reserve
'detachment' is the definition.
(thesaurus)
'military work' is the wordplay.
'military' becomes 'RE' (Royal Engineers).
'work' becomes 'serve' (serving is a kind of working).
're'+'serve'='RESERVE'
'of soldiers to do' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for reserve that I've seen before include "Savings - book" , "Keep back - book" , "put to one side" , "Order" , "some distance" .)