To run away before one final check is valid (10)
I believe the answer is:
legitimate
'valid' is the definition.
('legitimate' can be similar in meaning to 'valid')
'to run away before one final check' is the wordplay.
'to run away' becomes 'legit' (as in 'leg it').
'before' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'final check' becomes 'mate' (checkmate, the final check in a game of chess).
'legit'+'i'+'mate'='LEGITIMATE'
'is' acts as a link.
(Other definitions for legitimate that I've seen before include "Permitted" , "It's admissible" , "Lawful" , "Legal" , "In accordance with the rules" .)