Sink to bottom of heap in effect again (7)
I believe the answer is:
torpedo
'sink' is the definition.
(I've seen this in another clue)
'to bottom of heap in effect again' is the wordplay.
'bottom of' suggests the final letters.
'in' is an insertion indicator.
'effect again' becomes 'redo' ('do' can be a synonym for 'effect' so perhaps 'redo' is OK for 'effect again').
The final letter of 'heap' is 'p'.
'p' placed into 'redo' is 'rpedo'.
'to'+'rpedo'='TORPEDO'
(Other definitions for torpedo that I've seen before include "Explosive missile from submarine" , "Naval missile" , "Weapon travelling under water" , "Poor Ted (anag)" , "Trooped (anag.)" .)