Looking fit, Mark somersaults expending last bit of energy (5)
I believe the answer is:
toned
'looking fit' is the definition.
'toned' can be an answer for 'fit' (I've seen this in another clue). I'm unsure of the 'looking' bit.
'somersaults expending last bit of energy' is the wordplay.
I cannot really see how this works, but
'last' could be 't' (I've seen this in other clues) and 't' is found within the answer.
'bit' could be 'o' (resembles 0, a binary digit or bit) and 'o' is located in the answer.
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'mark' acts as a link.
Can you help me to learn more?
(Other definitions for toned that I've seen before include "Well-muscled" , "In good condition (of muscles or skin)" , "Having firm and well-developed muscles" , "Slightly tinted; firm-muscled" , "Noted (anag.)" .)