Young bird looked extremely sorrowful during farewell (7)
I believe the answer is:
gosling
'young bird' is the definition.
(I've seen this before)
'extremely sorrowful during farewell' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'during' is an insertion indicator.
'farewell' becomes 'going' (I am not sure about this - if you are sure you should believe this answer much more).
'sorrowful' with its centre removed is 'sl'.
'sl' placed within 'going' is 'GOSLING'.
'looked' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for gosling that I've seen before include "Young goose" , "Little bird" , "Young bird" , "Young waterbird" .)