Sign of gratitude, having supplied skill (6)
I believe the answer is:
talent
'skill' is the definition.
(talent can mean a skill or ability)
'sign of gratitude having supplied' is the wordplay.
'sign of gratitude' becomes 'ta' ('ta' can mean thanks).
'having' is a charade indicator (letters next to each other).
'supplied' becomes 'lent'.
'ta'+'lent'='TALENT'
(Other definitions for talent that I've seen before include "Natural ability for some activity" , "Natural aptitude or skill" , "Old coin for natural ability" , "Special ability, flair" , "Natural aptitude of an old coin" .)