Consider the river at Maidenhead (4)
I believe the answer is:
deem
'consider' is the definition.
(I know this)
'river at maidenhead' is the wordplay.
'river' becomes 'Dee' (River Dee).
'at' means one lot of letters go next to another.
'maidenhead' becomes 'm' (head letter of 'maiden').
'dee'+'m'='DEEM'
'the' is the link.
(Other definitions for deem that I've seen before include "Believe" , "Regard, consider" , "Judge, think, consider" , "Card-table felt" , "Adjudge" .)