Specify five — just half ten (3)
I believe the answer is:
fix
'specify' is the definition.
(I know that specify can be written as fix)
'five just half ten' is the wordplay.
'just half' means to take half the letters.
'ten' becomes 'X' (Roman numeral).
'five' halved is 'fi'.
'fi'+'x'='FIX'
(Other definitions for fix that I've seen before include "mess" , "Sort out" , "Addict's dose of drug" , "Fasten" , "Mend" .)