Harp on about fragment found in wreck (3,2,2)
I believe the answer is:
rub it in
'harp on about' is the definition.
(both can mean to repeatedly mention something)
'fragment found in wreck' is the wordplay.
'fragment' becomes 'bit' (bit is a kind of fragment).
'found in' means one lot of letters goes inside another.
'wreck' becomes 'ruin' (I've seen this before).
'bit' put into 'ruin' is 'RUB IT IN'.
(Other definitions for rub it in that I've seen before include "Remind someone maliciously of an embarrassment" , "Remind person of unpleasant fact - during massage?" , "Remind or taunt someone about unpleasant fact" , "Unkindly emphasise" , "Harp on something distasteful to person not wishing to be reminded of it" .)