Pooh, investing first of his old pennies, made a nicker (8)
I believe the answer is:
whinnied
'made a nicker' is the definition.
(a nicker is a whinnying sound)
'pooh investing first of his old pennies' is the wordplay.
'pooh' becomes 'Winnie' (Winnie the Pooh in stories by AA Milne).
'investing' means one lot of letters goes inside another (inserted letters are taken in or invested).
'first of his' becomes 'h' (1st letter of 'his').
'old pennies' becomes 'd' (historical abbreviation for pennies).
'winnie' going around 'h' is 'whinnie'.
'whinnie'+'d'='WHINNIED'
(Another definition for whinnied that I've seen is " Neighed".)