Source of magnesium found in drink from reservoir (4)
I believe the answer is:
sump
'reservoir' is the definition.
(sump is a kind of reservoir)
'source of magnesium found in drink' is the wordplay.
'source of' says to take the initial letters.
'found in' means one lot of letters goes inside another.
'drink' becomes 'sup' (I've seen this before).
The initial letter of 'magnesium' is 'm'.
'm' inserted within 'sup' is 'SUMP'.
'from' is the link.
(Other definitions for sump that I've seen before include "Reservoir for oil etc" , "Crankcase reservoir" , "Engine's oil reservoir" , "Cesspool" , "It receives the engine oil" .)