Nominally a unit (4)

I believe the answer is:

abel

I'm a little stuck... Click here to teach me more about this clue!

'nominally a unit' is the definition.
I can't tell whether this definition defines the answer.

'a unit' is the wordplay.
'unit' becomes 'bel' (bel is a kind of unit**).
'a'+'bel'='ABEL'

Can you help me to learn more?

(Other definitions for abel that I've seen before include "Cain's 11" , "tragic figure" , "One of the first children" , "Victim of 11" , "Old Testament brother" .)

I've seen this clue in the Evening Standard.

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