Slender girls's first object (5)
I believe the answer is:
thing
'object' is the definition.
(an object is a thing or item)
'slender girls's first' is the wordplay.
'slender' becomes 'thin' (similar in meaning).
'girls's first' becomes 'g' (1st letter of 'girls').
'thin'+'g'='THING'
(Other definitions for thing that I've seen before include "Inanimate object" , "Night (anag)" , "cup of tea" , "Unspecified object" , "Object; gadget" .)