Firm Republican guarded by ardent squad (6)
I believe the answer is:
cohort
'squad' is the definition.
(thesaurus)
'firm republican guarded by ardent' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'republican' becomes 'R' (abbreviation).
'guarded by' is an insertion indicator.
'ardent' becomes 'hot' (synonyms).
'r' inserted within 'hot' is 'hort'.
'co'+'hort'='COHORT'
(Other definitions for cohort that I've seen before include "Group of those of approximately the same age -- Roman military unit" , "Band of warriors" , "team of soldiers" , "Band of fighters" , "Unit of Roman soldiers - group of people of same age" .)