Ultimately, this taxi's not one loved by unions (4)
I believe the answer is:
Here is my best explanation:
'ultimately this taxi's not one loved by unions' is the definition.
The definition suggests a singular noun which matches the answer.
'ultimately this taxi's' is the wordplay.
'ultimately' says to take the final letters.
'taxi' becomes 'cab' (synonyms).
The last letter of 'this' is 's'.