Yearned after length of yarn, made a mistake right away (8)
I believe the answer is:
hankered
'yearned' is the definition.
('hanker' can be a synonym of 'yearn')
'length of yarn made a mistake right away' is the wordplay.
'length of yarn' becomes 'hank' (I've seen this in another clue).
'made a mistake' becomes 'erred' ('err' can be a synonym of 'make a mistake').
'right' becomes 'r' (common abbreviation).
'away' indicates named letters should be taken away.
'erred' with 'r' removed is 'ered'.
'hank'+'ered'='HANKERED'
'after' is the link.
(Another definition for hankered that I've seen is " Longed, yearned".)