One Liberal Democrat finally breaking rank (11)
I believe the answer is:
intolerable
'rank' is the definition.
(thesaurus)
'one liberal democrat finally breaking' is the wordplay.
'finally' indicates one should take the final letters.
'breaking' is an anagram indicator.
The final letter of 'democrat' is 't'.
'one'+'liberal'+'t'='oneliberalt'
'oneliberalt' anagrammed gives 'INTOLERABLE'.
(Other definitions for intolerable that I've seen before include "Unable to be endured" , "Unbearable" , "The last straw" .)