Unforeseen difficulty in mine before collapse (7)
I believe the answer is:
pitfall
'unforeseen difficulty' is the definition.
(I've seen this before)
'mine before collapse' is the wordplay.
'mine' becomes 'pit' (down the pit).
'before' says to put letters next to each other.
'collapse' becomes 'fall' (I've seen this in another clue).
'pit'+'fall'='PITFALL'
'in' is the link.
(Other definitions for pitfall that I've seen before include "Trap in the form of a concealed hole" , "Unexpected difficulty or danger" , "Unforeseen difficulty" , "Hidden danger or difficulty" , "Hazard" .)