Mine's on decline - that's the hidden danger (7)
I believe the answer is:
pitfall
'the hidden danger' is the definition.
(I've seen this before)
'mine's on decline' is the wordplay.
'mine' becomes 'pit' (down the pit).
'on' is a charade indicator (letters next to each other).
'decline' becomes 'fall' ('fall' can be a synonym of 'decline').
'pit'+'fall'='PITFALL'
'that's' is the link.
(Other definitions for pitfall that I've seen before include "Unforeseen difficulty" , "Trap in the form of a concealed hole" , "Unexpected problem" , "Hidden danger or difficulty" , "Hazard" .)