Beautiful girl, if undersized (5)
I believe the answer is:
model
'undersized' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I can't see?
'beautiful girl' is the wordplay.
I cannot really understand how this works, but
'girl' could be 'del' and 'del' is found within the answer.
The remaining letters 'mo' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'if' acts as a link.
Can you help me to learn more?
(Other definitions for model that I've seen before include "Fashion - prototype" , "Exemplary - mannequin" , "She parades on the catwalk" , "She saunters on the catwalk" , "(Person functioning as) example" .)