Uncertainty following start of dispute with union is spreading (9)

'spreading' is the definition.
(diffusion is a kind of spreading)

'uncertainty following start of dispute with union' is the wordplay.
'uncertainty' becomes 'if' (I've seen this in other clues).
'following' says to put letters next to each other.
'start of' indicates taking the first letters.
'with union' becomes 'fusion' (fusion is a kind of union. I am not sure about the 'with' bit.**).
The first letter of 'dispute' is 'd'.
'if' put after 'd' is 'dif'.
'dif'+'fusion'='DIFFUSION'

'is' is the link.

Can you help me to learn more?