Calculate, at the start of the week, that they will send for them (6)
I believe the answer is:
summon
'they will send for them' is the definition.
The definition suggests an adverb but the answer is not.
'calculate at the start of the week' is the wordplay.
'calculate at' becomes 'sum' (I can't explain this - if you can you should give a lot more credence to this answer).
'the start of the week' becomes 'mon' (Monday).
'sum'+'mon'='SUMMON'
'that' is the link.
Can you help me to learn more?
(Other definitions for summon that I've seen before include "Convene" , "Order to come" , "Send for" , "Request appearance of" , "Call upon" .)