First of haunting autumnal images Keats used to write poem (5)
I believe the answer is:
haiku
'poem' is the definition.
(haiku is a kind of poem)
'first of haunting autumnal images keats used' is the wordplay.
'first of' suggests taking the first letters.
The initial letters of 'haunting autumnal images keats used' is 'HAIKU'.
'to write' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for haiku that I've seen before include "Eastern poetry" , "Japanese 3-line poem of 17 syllables" , "Jap 17-syllable poem" , "Stylised Japanese poem of 17 syllables" , "Japanese form of poem" .)