An aid to circulation (4)
I believe the answer is:
vein
'circulation' is the definition.
(I know that vein is a type of blood vessel)
'an aid' is the wordplay.
I cannot quite understand how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is found in the answer.
The remaining letters 'ven' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for vein that I've seen before include "Blood-carrying vessel" , "part of body" , "Vessel for claret" , "Speaker's useless" , "Tube that carries blood to the heart" .)