Custodian, punched by six-footer on drugs, swore (10)
I believe the answer is:
guaranteed
'swore' is the definition.
(I've seen this before)
'custodian punched by six-footer on drugs' is the wordplay.
'custodian' becomes 'guard' (I've seen this before).
'punched by' is an insertion indicator (some letters punch a hole in others).
'six footer' becomes 'ant' (an ant has six feet).
'on' says to put letters next to each other.
'drugs' becomes 'ee' (an abbreviation for ecstasy written twice).
'ant'+'ee'='antee'
'guard' going around 'antee' is 'GUARANTEED'.
(Other definitions for guaranteed that I've seen before include "protected" , "Supported" , "Ensured" , "Covered against failure" , "Promised" .)