Redhead assumes local drug will be unpleasantly severe (5)
I believe the answer is:
harsh
'be unpleasantly severe' is the definition.
(I've seen this in another clue)
'redhead assumes local drug' is the wordplay.
I cannot quite see how this works, but
'redhead' could be 'r' (the head letter of red) and 'r' is located in the answer.
'drug' could be 'h' (H is an example) and 'h' is located in the answer.
The remaining letters 'ash' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'will' is the link.
Can you help me to learn more?
(Other definitions for harsh that I've seen before include "'Severe, rough (5)'" , "Not relenting" , "Abrasive, rough or unkind" , "Severe - unkind" , "Strident" .)