Disparage article worn by true nerd at sea (9)
I believe the answer is:
underrate
'disparage' is the definition.
(thesaurus)
'article worn by true nerd at sea' is the wordplay.
'article' becomes 'a' (indefinite article).
'worn by' indicates putting letters inside (I've seen this in other clues).
'at sea' indicates an anagram.
'true'+'nerd'='truenerd'
'truenerd' anagrammed gives 'underrte'.
'a' going inside 'underrte' is 'UNDERRATE'.
(Other definitions for underrate that I've seen before include "Not value enough" , "Value insufficiently, think too little of" , "Have too low an opinion of" , "Fail to recognise true value of" , "Think too little of, fail to appreciate" .)