Daft old chap interrupts Mike (7)
I believe the answer is:
moronic
'daft' is the definition.
(thesaurus)
'old chap interrupts mike' is the wordplay.
'old' becomes 'o' (abbreviation).
'chap' becomes 'ron' (short for Ronald).
'interrupts' means one lot of letters goes inside another.
'mike' becomes 'mic' (I've seen this in other clues).
'o'+'ron'='oron'
'oron' inserted into 'mic' is 'MORONIC'.
(Other definitions for moronic that I've seen before include "Foolish" , "really stupid" , "ROM coin (anag.)" , "Omicron (anag) - stupid" , "Mindless" .)