She lives on air, partly! (4)
I believe the answer is:
iris
'she' is the definition.
(I've seen this before)
'lives on air partly' is the wordplay.
'lives' becomes 'is' ('be' can be a synonym of 'live').
'on' means one lot of letters go next to another.
'air partly' becomes 'ir' (I can't justify this - if you can you should give a lot more credence to this answer).
'is' put after 'ir' is 'IRIS'.
Can you help me to learn more?
(Other definitions for iris that I've seen before include "who keeps pupil in?" , "Flower - rainbow" , "Circular coloured membrane in the eye" , "- - Murdoch, novelist" , "Eye diaphragm" .)