Lock up in Portlaoise behind unit near motorway (8)

I believe the answer is:

imprison

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

'lock up' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for imprison that I've seen before include "Put in gaol" , "shut up" , "Detain" , "Incarcerate" , "Purveyor of blooms" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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