Relentless old men with capacity to support home (10)
I believe the answer is:
inexorable
'relentless' is the definition.
(synonyms)
'old men with capacity to support home' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'men' becomes 'or' (military abbreviation for Other Ranks).
'with capacity' becomes 'able'.
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'home' becomes 'in' ('he's home' can mean 'he's in').
'ex'+'or'+'able'='exorable'
'exorable' after 'in' is 'INEXORABLE'.
(Other definitions for inexorable that I've seen before include "Relentless" , "Inescapable" , "Unstoppable" , "Unrelenting" , "Impossible to stop" .)