Discharge of soldiers in a division (9)
I believe the answer is:
secretion
'discharge' is the definition.
(thesaurus)
'soldiers in a division' is the wordplay.
'soldiers' becomes 'RE' (Royal Engineers).
'in' means one lot of letters goes inside another.
'a division' becomes 'section' (synonyms).
're' placed into 'section' is 'SECRETION'.
'of' is the link.
(Other definitions for secretion that I've seen before include "Product of glandular activity" , "Discharge - is on Crete (anag)" .)