Hold back army and strengthen with further support (9)
I believe the answer is:
reinforce
'strengthen with further support' is the definition.
'reinforce' can be an answer for 'strengthen' (reinforcing is a kind of strengthening). I'm not certain of the 'with further support' bit.
'hold back army' is the wordplay.
'hold back' becomes 'rein' (I've seen this before).
'army' becomes 'force' (I've seen this before).
'rein'+'force'='REINFORCE'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for reinforce that I've seen before include "Strengthen or support with confrerie" , "give added support" , "increase support" , "Give added strength to" , "Buttress" .)