Gloat when piece is in ruin (3,2,2)
I believe the answer is:
rub it in
'gloat' is the definition.
(I've seen this in another clue)
'piece is in ruin' is the wordplay.
'piece' becomes 'bit' (bit is a kind of piece).
'is in' indicates putting letters inside.
'bit' inserted inside 'ruin' is 'RUB IT IN'.
'when' acts as a link.
(Other definitions for rub it in that I've seen before include "Remind or taunt someone about unpleasant fact" , "Harp on to someone about something of which he does not wish to be reminded" , "Keep saying 'I told you so'" , "Continue to remind one of a mistake" , "Remind person of unpleasant fact - during massage?" .)