Girl in form is sixteen (4)
I believe the answer is:
miss
'girl' is the definition.
('miss' can be a synonym of 'girl')
'in form is sixteen' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MISS' is hidden in the letters of 'form is sixteen'.
(Other definitions for miss that I've seen before include "Fail to catch" , "Feel the absence of" , "Fail to hit; girl" , "Girl - bad shot" , "Young girl" .)