Could be young and strong (6)
I believe the answer is:
mighty
'strong' is the definition.
(thesaurus)
'could be young' is the wordplay.
'could' becomes 'might' ('might' can be a synonym of 'could').
'be young' becomes 'y'.
'might'+'y'='MIGHTY'
'and' acts as a link.
(Other definitions for mighty that I've seen before include "Powerful and great" , "Large" , "Arrogant people are often described as high and . . . . . ." , "Having great strength" , "Powerful - huge" .)