Staff drowning feeble king in watercourse (8)
I believe the answer is:
millrace
'watercourse' is the definition.
(course of water powering a mill)
'staff drowning feeble king' is the wordplay.
'staff' becomes 'mace' (mace is a kind of staff).
'drowning' indicates putting letters inside (inserted letters are drowned or absorbed).
'feeble' becomes 'ill' (both can mean unwell).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'ill'+'r'='illr'
'mace' enclosing 'illr' is 'MILLRACE'.
'in' is the link.
(Other definitions for millrace that I've seen before include "Channel for water driving wheel" , "current providing power" , "Channel leading to water wheel" , "Channel for the water current that turns a wheel" , "Flow of water" .)