Soldiers primarily seen inside compound (5)
I believe the answer is:
resin
'compound' is the definition.
(resin is a kind of compound)
'soldiers primarily seen inside' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'primarily' says to take the initial letters.
'inside' becomes 'in'.
The initial letter of 'seen' is 's'.
're'+'s'+'in'='RESIN'
(Other definitions for resin that I've seen before include "stuff that's sticky" , "Unprocessed pine secretion" , "It's obtained from tree-sap" , "Rinse out this substance from trees" , "Contributor to plastics industry" .)