A pair of queens to keep providing a water source (7)
I believe the answer is:
aquifer
'a water source' is the definition.
(I've seen this before)
'a pair of queens to keep providing' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'providing' could be 'if' (eg 'providing it's sunny' means 'if it's sunny') and 'if' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for aquifer that I've seen before include "Water-bearing stratum of permeable rock" , "Water-holding rock" , "Water-carrying layer of rock" , "Water-bearing layer" , "Rock containing water" .)