Sons engage in robust struggle (6)
I believe the answer is:
hassle
'struggle' is the definition.
(I know that hassle is a type of combat)
'sons engage in robust' is the wordplay.
'sons' becomes 'ss' (genealogical abbreviation for son written twice).
'engage in' indicates putting letters inside.
'robust' becomes 'hale' (in good health).
'ss' going into 'hale' is 'HASSLE'.
(Other definitions for hassle that I've seen before include "Difficulty - dispute" , "Pester (colloq.)" , "Modern word for bother and annoyance" , "Harass; pester" , "badger" .)