Number one is unique (4)

I believe the answer is:

lone

'unique' is the definition.
(thesaurus)

'number one' is the wordplay.
'number' becomes 'l' (L is an example**).
'l'+'one'='LONE'

'is' is the link.

(Other definitions for lone that I've seen before include "He likes to operate by himself" , "Companionless" , "Solitary, without companions" , "Unaccompanied" , "Solitary or single" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app