A long second act unfolds like a slow, geological process. (10)

'geological process' is the definition.
(I know that glaciation is a type of geological process)

'a long second act unfolds like a slow' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
'long' could be 'l' and 'l' is present in the answer.
'second act' could be 'c' (2nd letter of 'act') and 'c' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?