A corps or 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'

'or' acts as a link.

(Other definitions for are that I've seen before include "is for more than one" , "Unit of land measure equal to 100 square metres" , "(They/you/we) exist" , "Era (anag.)" , "(We/you/they) exist" .)

I've seen this clue in the Evening Standard.

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