Stylish marker pen that is missing (5)
I believe the answer is:
sharp
'stylish' is the definition.
(thesaurus)
'marker pen that is missing' is the wordplay.
'marker pen' becomes 'sharpie' (brand of permanent marker).
'that is' becomes 'ie' (synonyms).
'missing' indicates named letters should be taken away.
'sharpie' with 'ie' removed is 'SHARP'.
(Other definitions for sharp that I've seen before include "needing strings slackened?" , "Acidic; smart and stylish" , "Cutting; acidic" , "British musician" , "exactly on time" .)