Cautious and tired, losing energy (4)
I believe the answer is:
wary
'cautious' is the definition.
(similar in meaning)
'tired losing energy' is the wordplay.
'tired' becomes 'weary' (similar in meaning).
'losing' indicates named letters should be taken away.
'energy' becomes 'e' (physics symbol as in E=mc2).
'weary' with 'e' taken away is 'WARY'.
'and' acts as a link.
(Other definitions for wary that I've seen before include "Displaying caution" , "Circumspect" , "Watchful" , "Guarded" , "Cautious, suspicious" .)