Underwear designer -- his pants having poor taste (8)
I believe the answer is:
brackish
'underwear designer' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'his pants having poor taste' is the wordplay.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'having' is an insertion indicator.
'poor taste' becomes 'brack' (I can't justify this - if you can you should believe this answer much more).
'his' anagrammed gives 'ish'.
'ish' enclosing 'brack' is 'BRACKISH'.
Can you help me to learn more?
(Other definitions for brackish that I've seen before include "Sightly salty, as water in estuaries" , "rather salty" , "Sightly salty, as in river estuaries" , "Somewhat salty" , "unpalatable" .)