A corps is a unit (3)

I believe the answer is:

are

'unit' is the definition.
(are is a kind of unit)

'a corps' is the wordplay.
'corps' becomes 'RE' (Royal Engineers army regiment).
'a'+'re'='ARE'

'is a' is the link.

(Other definitions for are that I've seen before include "(They/you/we) exist" , "Exist; square measure" , "Is, in the plural" , "is for more than one" , "(We/they/you) exist" .)

I've seen this clue in the Evening Standard.

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