Union dead in ground on their own (7)
I believe the answer is:
unaided
'own' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?
'union dead in ground on' is the wordplay.
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'on' says to put letters next to each other.
'dead'+'in'='deadin'
'deadin' is an anagram of 'naided'.
'u'+'naided'='UNAIDED'
'their' is the link.
Can you help me to learn more?
(Other definitions for unaided that I've seen before include "Without help or support" , "having received zero help" , "Quite without assistance" , "Dun aide being without help" , "Without help or assistance" .)