Check trousers stained at front with sticky substance (5)
I believe the answer is:
resin
'sticky substance' is the definition.
(I've seen this before)
'check trousers stained at front' is the wordplay.
'check' becomes 'rein' (both can mean to control or limit).
'trousers' means one lot of letters goes inside another (to trouser can mean to contain or hold).
'at front' indicates taking the first letters.
The first letter of 'stained' is 's'.
'rein' going around 's' is 'RESIN'.
'with' acts as a link.
(Other definitions for resin that I've seen before include "stuff that's sticky" , "Sticky stuff from pine trees" , "Myrrh, for instance" , "Glue" , "Sticky exudation from trees" .)