One who sells dear, perhaps, in extremes of terror (6)
I believe the answer is:
trader
'one who sells' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'dear perhaps in extremes of terror' is the wordplay.
'perhaps' indicates an anagram.
'in' is an insertion indicator.
'extremes of' suggests removing the centre (only the letters at the extremes of the word).
'terror' with its middle taken out is 'tr'.
'dear' anagrammed gives 'rade'.
'rade' placed inside 'tr' is 'TRADER'.
Can you help me to learn more?
(Other definitions for trader that I've seen before include "Red tar (anag)" , "Tarred the dealer badly" , "Dealer who buys and sells" , "Dealer, monger" , "One in business" .)