Various units entering flat (13)
I believe the answer is:
uninteresting
'flat' is the definition.
(I've seen this before)
'various units entering' is the wordplay.
'various' indicates anagramming the letters.
'units'+'entering'='unitsentering'
'unitsentering' anagrammed gives 'UNINTERESTING'.
(Other definitions for uninteresting that I've seen before include "flat" , "Not arousing curiosity" , "Dull, boring" .)