Cook slowly leaves M&S having returned sandwiches (5)
I believe the answer is:
steam
'cook slowly' is the definition.
'steam' can be an answer for 'cook' (steaming is a kind of cooking). I am not sure about the 'slowly' bit.
'leaves ms having returned sandwiches' is the wordplay.
'leaves' becomes 'tea' (as in tea leaves).
'having returned' says the letters should be written in reverse.
'sandwiches' is an insertion indicator (inserted letters are sandwiched in).
'ms' reversed gives 'sm'.
'tea' inserted inside 'sm' is 'STEAM'.
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Type of power" , "Gas from boiliing water" , "Water vapour; impetus" , "Vapour; energy" , "Cook" .)