Element of watery discharge swamping part of UK (7)
I believe the answer is:
rhenium
'element' is the definition.
(rhenium is a kind of element)
'watery discharge swamping part of uk' is the wordplay.
'watery discharge' becomes 'rheum' (watery discharge in the eyes).
'swamping' means one lot of letters goes inside another (swamp can mean to surround or smother).
'part of uk' becomes 'NI' (Northern Ireland).
'rheum' enclosing 'ni' is 'RHENIUM'.
'of' is the link.
(Other definitions for rhenium that I've seen before include "Metal" , "Chemical" , "Element (Re)" .)