Strike-breaker up north finds mountain lake (4)
I believe the answer is:
tarn
'mountain lake' is the definition.
(I've seen this before)
'strike-breaker up north' is the wordplay.
'strike-breaker' becomes 'rat' (I've seen this before).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'north' becomes 'n' ('N' can be a synonym of 'north').
'rat' backwards is 'tar'.
'tar'+'n'='TARN'
'finds' acts as a link.
(Other definitions for tarn that I've seen before include "High water" , "Scottish mountain lake" , "Small mountain lake" , "Mountain pool: French department" .)