Hidden danger behind returning medication boxes (7)
I believe the answer is:
pitfall
'hidden danger' is the definition.
(pitfalls are unexpected dangers)
'behind returning medication boxes' is the wordplay.
'behind' becomes 'aft' (aft means towards or at the end of a ship).
'returning' shows that the letters should be reversed in order.
'medication' becomes 'pill' (pill is a kind of medication).
'boxes' is an insertion indicator (the outside letters must 'box' inside letters in).
'aft' back-to-front is 'tfa'.
'tfa' inserted within 'pill' is 'PITFALL'.
(Other definitions for pitfall that I've seen before include "Unforeseen difficulty" , "Trap in the form of a concealed hole" , "Unexpected difficulty or danger" , "Hazard" , "Unsuspected danger or difficulty - for miner?" .)