Prayer penned by dude for qualification (6)
I believe the answer is:
caveat
'for qualification' is the definition.
(I've seen this in another clue)
'prayer penned by dude' is the wordplay.
'prayer' becomes 'ave' (as in Ave Maria).
'penned by' means one lot of letters goes inside another (inserted letters are penned in).
'dude' becomes 'cat' (American slang for man, esp in Jazz).
'ave' placed within 'cat' is 'CAVEAT'.
(Other definitions for caveat that I've seen before include "'Warning, proviso (6)'" , "Warning (of conditions)" , "Vacate (anag) - warning" , "A notice or warning" , "Caution - qualification" .)