Cheat's fed up with a drug -- almost strange (7)
I believe the answer is:
defraud
'cheat's' is the definition.
(defrauding is a kind of cheating)
'fed up with a drug almost strange' is the wordplay.
'fed up' becomes 'def' ('fed' going up in a down clue).
'with' means one lot of letters goes inside another.
'almost' means to remove the last letter (most of the word but not all of it).
'strange' indicates anagramming the letters.
'drug' with its last letter taken away is 'dru'.
'dru' anagrammed gives 'rud'.
'a' put into 'rud' is 'raud'.
'def'+'raud'='DEFRAUD'
(Other definitions for defraud that I've seen before include "Cheat, deceive" , "Swindle" , "Get money from (someone) by deception" , "Illegally get money from (someone)" , "Diddle" .)