Short flight is hard work (3)
I believe the answer is:
hop
'short flight' is the definition.
(I've seen this in another clue)
'hard work' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'work' becomes 'op' (abbreviation for opus).
'h'+'op'='HOP'
'is' is the link.
(Other definitions for hop that I've seen before include "Cone which flavours beer" , "A short journey" , "Leap on one leg" , "Dancing party (informal)" , "Jump on one foot" .)