Prison term that is finding one under par (6)
I believe the answer is:
birdie
'one under par' is the definition.
(I've seen this in another clue)
'prison term that is' is the wordplay.
'prison term' becomes 'bird' (bird can mean time served in prison).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'bird'+'ie'='BIRDIE'
'finding' is the link.
(Other definitions for birdie that I've seen before include "Good hole" , "Golf score of one stroke under par" , "One under in 20 across" , "One under par - watch it!" , "Golfing score better than par" .)