Haggard work crew filling up very thin (5)
I believe the answer is:
sheer
'very thin' is the definition.
(I've seen this in another clue)
'haggard work crew filling up' is the wordplay.
'haggard work' becomes 'she' (I've seen this in another clue).
'filling' says to take the centre (the letters which fill the word).
'up' says the letters should be written backwards (in down clue: letters go upwards).
The central letters of 'crew' are 're'.
're' in reverse letter order is 'er'.
'she'+'er'='SHEER'
(Other definitions for sheer that I've seen before include "Complete" , "Downright - deviation" , "Absolutely - fine - straight up" , "sudden change of direction" , "total" .)