Main hazard in Britain: Tories like Rees-Mogg! (4)
I believe the answer is:
berg
'main hazard' is the definition.
(iceberg - main can mean the sea)
'britain tories like rees-mogg' is the wordplay.
'britain' becomes 'B' (common abbreviation as in GB).
'tories like rees-mogg' becomes 'ERG' (Jacob Rees-Mogg is part of European Research Group).
'b'+'erg'='BERG'
'in' is the link.
(Other definitions for berg that I've seen before include "South African mountain - Austrian composer" , "feature of icy waters" , "famously cut short a transatlantic journey" , "South African hill - Austrian composer" , "Possible shipping hazard" .)