Sticky stuff keeping son in check? (5)
I believe the answer is:
resin
'sticky stuff' is the definition.
(I've seen this before)
'son in check?' is the wordplay.
'son' becomes 's'.
'in' is an insertion indicator.
'check?' becomes 'rein' (both can mean to control or limit).
's' inserted within 'rein' is 'RESIN'.
'keeping' acts as a link.
(Other definitions for resin that I've seen before include "hashish, say" , "Synthetic compound" , "Sap product" , "Organic polymer used in varnishes, adhesives etc." , "Sticky stuff oozing from trees" .)