Complete agreement with leprechaun is only partial (6)
I believe the answer is:
unison
'complete agreement' is the definition.
(unison can mean total agreement or harmony)
'leprechaun is only partial' is the wordplay.
'partial' says the answer is hidden in the clue.
'UNISON' is hidden within 'leprechaun is only'.
'with' is the link.
(Other definitions for unison that I've seen before include "all players directed to the same pitch" , "Agreement or concord, as with choir" , "Concordance" , "Harmony - public services' trade union" , "Complete agreement, harmony" .)