Everyone can see that Carol's on drugs (5)
I believe the answer is:
using
'on drugs' is the definition.
(I've seen this before)
'everyone can see that carol's' is the wordplay.
'everyone can see' becomes 'U' (as in a Universal film certificate).
'that carol' becomes 'sing' (carol can mean to sing carols).
'u'+'sing'='USING'
(Other definitions for using that I've seen before include "Putting to a particular purpose" , "taking drugs" , "Exploiting for one's own advantage" , "addiction" , "having recourse to" .)