Teach girl about being inactive (9)
I believe the answer is:
lethargic
'inactive' is the definition.
(associated in meaning)
'teach girl about' is the wordplay.
'about' indicates an anagram.
'teach'+'girl'='teachgirl'
'teachgirl' is an anagram of 'LETHARGIC'.
'being' is the link.
(Other definitions for lethargic that I've seen before include "Enervated" , "reluctant to move" , "Lacking energy/enthusiasm" , "Lacking in energy or vitality" , "Listless" .)