Be prone to illness? (3,2)
I believe the answer is:
lie up
'prone to illness?' is the definition.
I can't tell whether this definition defines the answer.
'be' is the wordplay.
I cannot really see how this works, but
'be' could be 'lie' (lying is a kind of being) and 'lie' is located in the answer.
The remaining letters 'up' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lie up that I've seen before include "Stay in bed" , "(Of ship) go into dock" , "out of commission" , "Berth" .)