Nominally a bit of a setback for ASLEF (4)
I believe the answer is:
elsa
'bit of a setback for aslef' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'nominally a' is the wordplay.
'nominally' becomes 'els' (I can't justify this - if you can you should believe this answer much more).
'els'+'a'='ELSA'
Can you help me to learn more?
(Other definitions for elsa that I've seen before include "___ Schiaparelli, Italian-born French fashion designer d. 1973" , "Famous lioness" , "Lioness (born free in Kenya)" , "Big cat" , "Girl" .)