Girl is crazy about underground leader (4)
I believe the answer is:
maud
'girl' is the definition.
'crazy about underground leader' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'about' indicates putting letters inside.
'leader' suggests taking the first letters.
The first letter of 'underground' is 'u'.
'mad' enclosing 'u' is 'MAUD'.
'is' is the link.
(Other definitions for maud that I've seen before include "Girl invited into garden (Tennyson)" , "Old song: ''Come into the garden . . . .''" , "Tennyson poem" , "Come into the garden, - (Tennyson)" , "Girl invited into the garden in song" .)