Lacking energy, teach girl to dance (9)
I believe the answer is:
lethargic
'lacking energy' is the definition.
(I've seen this before)
'teach girl to dance' is the wordplay.
'to dance' indicates anagramming the letters (letters must dance around).
'teach'+'girl'='teachgirl'
'teachgirl' anagrammed gives 'LETHARGIC'.
(Other definitions for lethargic that I've seen before include "lack of energy" , "Lacking energy/enthusiasm" , "without energy" , "languid" , "Apathetic, lacking energy" .)