To change shape, fiancee regularly used strong drug (8)
I believe the answer is:
morphine
'strong drug' is the definition.
'morphine' can be an answer for 'drug' (morphine is a kind of drug). I am not certain of the 'strong' bit.
'to change shape fiancee regularly' is the wordplay.
'to change shape' becomes 'morph' (I can't justify this - if you can you should believe this answer much more).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'fiancee' are 'ine'.
'morph'+'ine'='MORPHINE'
'used' is the link.
Can you help me to learn more?
(Other definitions for morphine that I've seen before include "Nip Homer in the bud and you'll get a powerful painkiller" , "Opium drug for relieving pain" , "Sedative derived from opium" , "Powerful pain-reliever" , "Pain-killing drug" .)