Show weariness returning from a tiring assignment (3)
I believe the answer is:
sag
'show weariness' is the definition.
(I've seen this in another clue)
'returning from a tiring assignment' is the wordplay.
'returning' is a reversal indicator.
'from' says the answer is hidden in the clue.
'gas' is hidden in the letters of 'tiring assignment'.
'gas' reversed gives 'SAG'.
(Other definitions for sag that I've seen before include "Give in the middle" , "Droop in the middle - it's gas" , "Give way under pressure" , "Sink (under load)" , "Sink under weight" .)