Flier, not fully fit, seen returning around part of airship (6)
I believe the answer is:
icarus
'flier' is the definition.
(I have seen 'Foolhardy flier ' mean 'icarus' so perhaps 'flier' could also mean 'icarus')
'seen returning around part of airship' is the wordplay.
'seen' becomes 'sui' (I can't justify this - if you can you should believe this answer much more).
'returning' says the letters should be written backwards.
'around' indicates putting letters inside.
'part of airship' becomes 'car' ('car' is part of 'airship').
'sui' back-to-front is 'ius'.
'ius' placed around 'car' is 'ICARUS'.
'not fully fit' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for icarus that I've seen before include "Ancient Greek who flew too close to the sun" , "One whose wings melted" , "Flier with wax wings" , "Ill-fated flier of myth" , "He was killed flying" .)