Strong and popular type of pain relief drug (7)
I believe the answer is:
intense
'strong' is the definition.
(similar in meaning)
'popular type of pain relief drug' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'type of pain relief' becomes 'tens' (I can't explain this - if you can you should give a lot more credence to this answer).
'drug' becomes 'e' (E is another name for ecstasy).
'in'+'tens'+'e'='INTENSE'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for intense that I've seen before include "Extreme - strong" , "Strongly-felt" , "Extremely emotional" , "Very concentrated or passionate" , "amplified?" .)