Ulster bus lane potentially is not fit for purpose (8)
I believe the answer is:
unusable
'not fit for purpose' is the definition.
(I've seen this before)
'ulster bus lane potentially' is the wordplay.
'ulster' becomes 'u' (this could be a standard abbreviation I've not previously seen).
'potentially' indicates an anagram (letters have the potential to spell a new word).
'bus'+'lane'='buslane'
'buslane' is an anagram of 'nusable'.
'u'+'nusable'='UNUSABLE'
'is' is the link.
Can you help me to learn more?
(Other definitions for unusable that I've seen before include "Of no practical benefit" , "Not fit for purpose" , "Of no utility" , "Not fit to employ" , "Not fit to be employed" .)