Different form of 'lain' in British English (7)
I believe the answer is:
unalike
'different' is the definition.
(not like something else)
'form of lain in british english' is the wordplay.
'form of' is an anagram indicator (another form of the same letters).
'in' is an insertion indicator.
'british' becomes 'uk' (Great Britain).
'english' becomes 'E' (abbreviation as in OED).
'lain' is an anagram of 'nali'.
'nali' put within 'uk' is 'unalik'.
'unalik'+'e'='UNALIKE'
(Other definitions for unalike that I've seen before include "Differing from each other" , "poor match" , "Incompatible" , "different" , "Dissimilar" .)