Poacher in river or lake breaking heart (7)
I believe the answer is:
tickler
'poacher in river' is the definition.
(trout tickling is a method of poaching)
'lake breaking heart' is the wordplay.
'lake' becomes 'l'.
'breaking' means one lot of letters goes inside another.
'heart' becomes 'ticker' (synonyms).
'l' inserted inside 'ticker' is 'TICKLER'.
'or' acts as a link.
(Other definitions for tickler that I've seen before include "One catching trout by hand" , "Difficult problem" , "Poker" , "trout farm employee?" , "puzzle" .)