Dieter ultimately has trouble with eccentric means to reduce fare? (8)
I believe the answer is:
railcard
'means to reduce fare?' is the definition.
(reduced train fare)
'dieter ultimately has trouble with eccentric' is the wordplay.
'ultimately' suggests the final letters.
'has' is a charade indicator (letters next to each other).
'trouble' becomes 'ail' (synonyms).
'with' is a charade indicator (letters next to each other).
'eccentric' becomes 'card' (card can mean an eccentric person).
The final letter of 'dieter' is 'r'.
'r'+'ail'+'card'='RAILCARD'
(Other definitions for railcard that I've seen before include "Warrant for cheaper travel" , "Pass for reduced train fares" .)