Savage insult which hampers rise of servicemen (3-6)
I believe the answer is:
cut-throat
'savage' is the definition.
'insult which hampers rise of servicemen' is the wordplay.
'insult' becomes 'cut' (cut is a kind of insult).
'which' becomes 'that'.
'hampers' indicates putting letters inside (hamper can mean to fasten).
'rise of' says the letters should be written in reverse.
'servicemen' becomes 'or' (abbreviation for Other Ranks).
'or' backwards is 'ro'.
'that' going around 'ro' is 'throat'.
'cut'+'throat'='CUT-THROAT'
(Other definitions for cut-throat that I've seen before include "Murderer - fiercely competitive" , "Ruthless in competition" , "Violent offender" , "Fiercely competitive - type of razor" .)