One fellow, I see, is sarcastic (6)
I believe the answer is:
ironic
'sarcastic' is the definition.
(similar in meaning)
'one fellow i see' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'fellow' could be 'ron' (masculine given name) and 'ron' is found within the answer.
'i' is found in the answer.
'see' could be 'c' (the word for the letter, according to Chambers) and 'c' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for ironic that I've seen before include "Humorously sarcastic or mocking" , "Dry (of wit)" , "not as you'd expect" , "Type of situation where one is mocked by fate" , "Wry, sarcastic" .)