Variable is a mere part in working out (9)
I believe the answer is:
parameter
'variable' is the definition.
(parameter can mean a variable or value)
'a mere part in working out' is the wordplay.
'in working out' indicates an anagram (the anagram must be worked out).
'a'+'mere'+'part'='amerepart'
'amerepart' is an anagram of 'PARAMETER'.
'is' acts as a link.
(Other definitions for parameter that I've seen before include "Limitation" , "Factor defining limits" , "One of a set of factors as conditions for operation of a system" , "Variable allowed to change" , "Quantifiable characteristic of a system" .)