Voracious guzzler originally quite thin (6)
I believe the answer is:
greedy
'voracious' is the definition.
(I've seen this before)
'guzzler originally quite thin' is the wordplay.
'originally' indicates taking the first letters.
'quite thin' becomes 'reedy' (I've seen this in another clue).
The first letter of 'guzzler' is 'g'.
'g'+'reedy'='GREEDY'
(Other definitions for greedy that I've seen before include "Having excessive desire for food or money" , "Having a voracious appetite" , "Wanting too much" , "Starving" , "Gluttonous" .)