Bulbs of 50 amps (5)
I believe the answer is:
lamps
'bulbs' is the definition.
(I know this)
'50 amps' is the wordplay.
'50' becomes 'l' (Roman numeral).
'l'+'amps'='LAMPS'
'of' acts as a link.
(Other definitions for lamps that I've seen before include "light fittings" , "Devices for giving light" , "Lights" .)