Ultimately, this taxi's not one loved by unions (4)
I believe the answer is:
scab
'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'.
's'+'cab'='SCAB'
Can you help me to learn more?
(Other definitions for scab that I've seen before include "Crust-like surface over a wound" , "sign of wound healing" , "Strike breaker" , "cover" , "Derogatory term for a strike-breaker" .)