Girl maintaining it's indispensable (5)
I believe the answer is:
vital
'indispensable' is the definition.
(similar in meaning)
'girl maintaining it's' is the wordplay.
'girl' becomes 'val' (short for Valerie).
'maintaining' means one lot of letters goes inside another.
'val' enclosing 'it' is 'VITAL'.
(Other definitions for vital that I've seen before include "Essential; lively" , "Energetic - displaying life" , "Most important, like certain body organs" , "Extremely important, essential to success" , "Crucially important, essential" .)