Support son immersed in sea and Scots river (8)
I believe the answer is:
mainstay
'support' is the definition.
(I know that mainstay is a type of support)
'son immersed in sea and scots river' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'immersed in' means one lot of letters goes inside another.
'sea' becomes 'main' (synonyms).
'and' means one lot of letters go next to another.
'scots river' becomes 'Tay' (River Tay in Scotland).
'main'+'tay'='maintay'
's' placed within 'maintay' is 'MAINSTAY'.
(Other definitions for mainstay that I've seen before include "Strong cable to secure mast, chief support" , "Strongest support" , "Pillar of strength" , "Linchpin, principle support" , "Strong cable that secures a mast, chief support generally" .)