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