A mobility aid (3)
I believe the answer is:
tow
'a mobility aid' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
This is all the clue.
Can you help me to learn more?
(Other definitions for tow that I've seen before include "Drag along behind, say barge" , "form of flax" , "Pull using cable" , "Pull along behind, say barge" , "Prepared fibres" .)