Might mark cryptic clues (6)
I believe the answer is:
muscle
'might' is the definition.
(both can mean strength)
'mark cryptic clues' is the wordplay.
'mark' becomes 'm'.
'cryptic' indicates anagramming the letters (a cryptic or obscure order of the letters).
'clues' with letters rearranged gives 'uscle'.
'm'+'uscle'='MUSCLE'
(Other definitions for muscle that I've seen before include "contractor" , "Bicep or tricep, for instance" , "One of the contractile organs of the body" , "It can contract in the body" , "Body tissue that expands and contracts" .)