Troops operational? Send in more of them (9)
I believe the answer is:
reinforce
'send in more of them' is the definition.
(reinforcing by sending more soldiers in)
'troops operational?' is the wordplay.
'troops' becomes 'RE' (Royal Engineers army regiment).
'operational?' becomes 'inforce' (I've seen this in another clue).
're'+'inforce'='REINFORCE'
(Other definitions for reinforce that I've seen before include "Strengthen or support with confrerie" , "Buttress" , "give added support" , "provide support" , "Give added strength to" .)