Slow students lack energy, to some extent (7)
I believe the answer is:
slacken
'slow' is the definition.
('slacken' can be a synonym of 'slow')
'students lack energy to some extent' is the wordplay.
'to some extent' indicates a hidden word.
'SLACKEN' is hidden in the letters of 'students lack energy'.
(Other definitions for slacken that I've seen before include "Loosen -- become less busy" , "Ease off (work)" , "reduce tension" , "relax" , "Make looser" .)