Body part is viewed under infrared (4)
I believe the answer is:
iris
'body part' is the definition.
(iris is a kind of body part)
'is viewed under infrared' is the wordplay.
I cannot really understand how this works, but
'is' is present in the answer.
'infrared' could be 'ir' (I've seen this in other clues) and 'ir' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for iris that I've seen before include "Flowers in the eye?" , "is 21[WOMAN], perhaps" , "bit of a looker" , "Lily" , "Woman" .)