Has very little energy for seven days, we're told (4)
I believe the answer is:
weak
'has very little energy' is the definition.
I can't judge whether this defines the answer.
'seven days we're told' is the wordplay.
'seven days' becomes 'week' (I've seen this in another clue).
'we're told' indicates a 'sounds like' (homophone) clue.
'week' sounds like 'WEAK'.
'for' is the link.
Can you help me to learn more?
(Other definitions for weak that I've seen before include "Undefended" , "Feeble or frail" , "Puny" , "Faint; frail" , "Wanting in physical strength" .)