Intelligence in Britain having to fall (5)
I believe the answer is:
brain
'intelligence' is the definition.
(brain is a kind of intelligence)
'britain having to fall' is the wordplay.
'britain' becomes 'b' (common abbreviation as in GB).
'having' means one lot of letters go next to another.
'to fall' becomes 'rain' (raining is a kind of falling**).
'b'+'rain'='BRAIN'
'in' acts as a link.
(Other definitions for brain that I've seen before include "It's in the skull" , "Thinking organ" , "Whack on the head" , "and 15 ac. The loss of experts through emigration" , "Thinking thing" .)