Wreck party after hill exercise (7)
I believe the answer is:
torpedo
'wreck' is the definition.
(I've seen this before)
'party after hill exercise' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'after' means one lot of letters go next to another.
'hill' becomes 'tor' (tor is a kind of hill).
'exercise' becomes 'pe' (Physical Education).
'tor'+'pe'='torpe'
'do' after 'torpe' is 'TORPEDO'.
(Other definitions for torpedo that I've seen before include "Naval missile" , "ruin" , "Poor Ted (anag)" , "Such as could sink" , "Trooped (anag.)" .)