Fastener - one of five on hand (4)
I believe the answer is:
nail
'fastener' is the definition.
(I know that nail is a type of fastener)
'one of five on hand' is the wordplay.
'one' becomes 'i' (Roman numeral).
'of five' becomes 'na' (I am not sure about this - if you are sure you should believe this answer much more).
'on' is a charade indicator (letters next to each other).
'hand' becomes 'L' (abbreviation for left, as in the left-hand side).
'i' put after 'na' is 'nai'.
'nai'+'l'='NAIL'
Can you help me to learn more?
(Other definitions for nail that I've seen before include "Hit this on the head to be bang-on" , "Fixing spike" , "Bring down" , "Hit this on the head to be dead right" , "Fastening device" .)