Interrogate as gaoler, extremely badly (5)
I believe the answer is:
grill
'interrogate' is the definition.
(I've seen this before)
'gaoler extremely badly' is the wordplay.
'extremely' means to remove the middle letters.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'gaoler' with its centre taken out is 'gr'.
'gr'+'ill'='GRILL'
'as' is the link.
(Other definitions for grill that I've seen before include "Cooking device" , "Question closely" , "Framework of metal bars" , "Intensely question; dish" , "pump" .)