Drug must hurt badly without hesitation (5,5)
I believe the answer is:
truth serum
'drug' is the definition.
(truth serum is a kind of drug)
'must hurt badly without hesitation' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'hesitation' becomes 'er' (sound made while hesitating - um, er).
'must'+'hurt'='musthurt'
'musthurt' with letters rearranged gives 'truthsum'.
'truthsum' placed around 'er' is 'TRUTH SERUM'.
(Other definitions for truth serum that I've seen before include "means to get whoppers out" , "that should get you talking!" .)