That man has check enclosed (6)
I believe the answer is:
herein
'enclosed' is the definition.
(I've seen this in another clue)
'that man has check' is the wordplay.
'that man' becomes 'he' (he refers to a man).
'has' is a charade indicator (letters next to each other).
'check' becomes 'rein' (both can mean to control or limit).
'he'+'rein'='HEREIN'
(Other definitions for herein that I've seen before include "Contained in this document" , "in this place" , "In this book" .)