Puffer train driver's tea ... (10)
I believe the answer is:
advertiser
'puffer' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'train driver's tea' is the wordplay.
'train' indicates an anagram (to train can mean to grow something into a shape).
'drivers'+'tea'='driverstea'
'driverstea' with letters rearranged gives 'ADVERTISER'.
Can you help me to learn more?
(Other definitions for advertiser that I've seen before include "Promoter" , "He promotes" , "Part of a newspaper's title" , "who does the plugging" , "One publicising a product (S, not Z)" .)