Girl is very young? Only at first (3)
I believe the answer is:
ivy
'girl' is the definition.
(woman's name)
'very young? only at first' is the wordplay.
I cannot quite understand how this works, but
'very' could be 'v' (abbreviation) and 'v' is present in the answer.
'young?' could be 'y' and 'y' is present in the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for ivy that I've seen before include "Four-year" , "Creeper plant" , "Evergreen climber" , "Climbing plant, associated with holly" , "Climbing-plant" .)