Pelt sailors with firm condiment (10)
I believe the answer is:
peppercorn
'condiment' is the definition.
(thesaurus)
'pelt sailors with firm' is the wordplay.
'pelt' becomes 'pepper' ('pepper' can be a synonym of 'pelt'**).
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'with' means one lot of letters go next to another.
'firm' becomes 'co' (a firm is a company).
'rn' after 'co' is 'corn'.
'pepper'+'corn'='PEPPERCORN'
(Other definitions for peppercorn that I've seen before include "providing little income for landlord" , "Nominal" , "See this ground by mill" , "(Of rent) notional" , "hot" .)