Introduction to Alceste not short in accompaniment (5)
I believe the answer is:
along
'in accompaniment' is the definition.
(synonyms)
'introduction to alceste not short' is the wordplay.
'introduction to' indicates taking the first letters.
'not short' becomes 'long' ('long' is the opposite to 'short').
The initial letter of 'alceste' is 'a'.
'a'+'long'='ALONG'
(Other definitions for along that I've seen before include "Moving in a constant direction on (something)" , "(Going) in company (with others)" , "From one end to the other" , "Beside - as groups often sing!" , "By the side of" .)