Keep a firm grip (10)
I believe the answer is:
stronghold
'keep' is the definition.
(a military stronghold)
'a firm grip' is the wordplay.
'a firm' becomes 'strong' (I've seen this before).
'grip' becomes 'hold' (synonyms).
'strong'+'hold'='STRONGHOLD'
(Other definitions for stronghold that I've seen before include "Keep" , "castle" , "Fortress" , "Defence" .)