Second-class flabby airship (5)
I believe the answer is:
blimp
'airship' is the definition.
(blimp is a kind of airship)
'second-class flabby' is the wordplay.
'second class' becomes 'b'.
'flabby' becomes 'limp' (I've seen this in another clue).
'b'+'limp'='BLIMP'
(Other definitions for blimp that I've seen before include "Small barrage balloon" , "David Low's cartoon colonel - airship used for publicity" , "full of hot air?" , "Elderly pompous reactionary person" , "needs cover when shooting" .)