An old reprobate (but still less than mature?) (3)
I believe the answer is:
rip
'an old reprobate but still' is the definition.
I can't tell whether this defines the answer.
'less than mature?' is the wordplay.
'less than' means to remove the last letter (not the full word).
'mature?' becomes 'ripe' ('ripe' can be similar in meaning to 'mature').
'ripe' with its final letter removed is 'RIP'.
Can you help me to learn more?
(Other definitions for rip that I've seen before include "Letters often seen on gravestones" , "Tear (e.g. cloth)" , "inscription on tomb" , "Roue's" , "Letters often carved on gravestones" .)