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" .)

I've seen this clue in The Globe and Mail, The Guardian and The Irish Independent.

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