They use fifty units of current (5)

I believe the answer is:

lamps

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'they use' is the definition.
Although both the answer and 'use' are nouns, they are not in the same form of the noun.

'fifty units of current' is the wordplay.
'fifty' becomes 'l' ('L' can be a synonym of 'fifty').
'units of current' becomes 'amps' (I've seen this before).
'l'+'amps'='LAMPS'

Can you help me to learn more?

(Other definitions for lamps that I've seen before include "light fittings" , "Lights" , "Devices for giving light" .)

I've seen this clue in the Evening Standard.

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