After heading off, arrive sore to swim in Rutland Water? (9)
I believe the answer is:
reservoir
'rutland water?' is the definition.
'reservoir' can be an answer for 'water?' (reservoir is a kind of water). I am unsure of the 'rutland' bit.
'after heading off arrive sore to swim' is the wordplay.
'after heading off' means to remove the first letter (the letter which heads the word is removed).
'to swim' indicates anagramming the letters.
'arrive'+'sore'='arrivesore'
'arrivesore' with its initial letter taken away is 'rrivesore'.
'rrivesore' is an anagram of 'RESERVOIR'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for reservoir that I've seen before include "Lake created to store water" , "River rose (anag.)" , "Lake for collecting and storing water" , "depot" , "Store for water etc" .)