Heart of gold in fellow (6)
I believe the answer is:
morale
'heart' is the definition.
(thesaurus)
'gold in fellow' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'in' indicates putting letters inside.
'fellow' becomes 'male' (I've seen this before**).
'or' going inside 'male' is 'MORALE'.
'of' is the link.
(Other definitions for morale that I've seen before include "Esprit de corps" , "Self-confidence" , "Degree of optimism (of troops, perhaps)" , "The spirit of a group or individual" , "Spirits" .)