Belt occasionally put on beneath a slip (7)
I believe the answer is:
blunder
'a slip' is the definition.
(thesaurus)
'belt occasionally put on beneath' is the wordplay.
'occasionally' indicates alternate letters.
'put on' means one lot of letters go next to another.
'beneath' becomes 'under' (I've seen this before).
The alternating letters of 'belt' are 'bl'.
'bl'+'under'='BLUNDER'
(Other definitions for blunder that I've seen before include "Stupid mistake" , "Embarrassing mistake" , "A stupid 1 down" , "Clumsy error" , "A clumsy mistake or movement" .)