Standing in train, get flustered (8)
I believe the answer is:
treating
'standing' is the definition.
(stand can mean to treat or provide with something)
'train get flustered' is the wordplay.
'flustered' is an anagram indicator.
'train'+'get'='trainget'
'trainget' is an anagram of 'TREATING'.
'in' is the link.
(Other definitions for treating that I've seen before include "Making no charge" , "Giving medical attention or paying for someone" , "Negotiating (with); handling" , "Standing a drink" , "Giving medical care to" .)