One of The Greens consumes one heady type of speed - the old type? (8)
I believe the answer is:
celerity
'speed the old type?' is the definition.
'celerity' can be an answer for 'speed' (I've seen this in another clue). I'm unsure of the 'the old type?' bit.
'one of the greens consumes one heady type' is the wordplay.
I cannot really see how this works, but
'one' could be 'er' (I've seen this in other clues) and 'er' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for celerity that I've seen before include "Dispatch" , "Expedition" , "showing a good turn of speed" , "Rapidity, speediness" , "Quickness" .)