Popular first pair of knickers to have (5)
I believe the answer is:
known
'popular' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'pair of knickers to have' is the wordplay.
'pair of knickers' becomes 'n' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'have' becomes 'know' (synonyms).
'n' after 'know' is 'KNOWN'.
'first' is the link.
Can you help me to learn more?
(Other definitions for known that I've seen before include "Apprehended mentally" , "Widely recognised" , "Famous" , "Experienced" , "Recognised, familiar" .)