Riddle has to be missing second shoe (7)
I believe the answer is:
trainer
'shoe' is the definition.
(I've seen this before)
'riddle has to be missing second' is the wordplay.
'riddle' becomes 'strainer' (riddle can mean a sieve or strainer).
'has to be missing' indicates named letters should be taken away.
'second' becomes 's' (common abbreviation).
'strainer' with 's' removed is 'TRAINER'.
(Other definitions for trainer that I've seen before include "Sports instructor" , "Soft shoe; simulator" , "Sports shoe" , "Machine that simulates the conditions of flying a plane" , "The odd thing an athlete may leave behind" .)