Poor Henry's taken in by a ruse at the same time (9)
I believe the answer is:
meanwhile
'at the same time' is the definition.
('meanwhile' can be a synonym of 'at the same time')
'poor henry's taken in by a ruse' is the wordplay.
'poor' becomes 'mean' ('mean' can be similar in meaning to 'poor'**).
'henry' becomes 'h' ('H' can be a synonym of 'henry').
'taken in by' indicates putting letters inside.
'a ruse' becomes 'wile' (I've seen this before).
'h' put into 'wile' is 'while'.
'mean'+'while'='MEANWHILE'
(Other definitions for meanwhile that I've seen before include "In the intervening time" , "In the interim" , "On the other hand" , "For the moment" , "in intervening period" .)