Seeing in the dark is often fun for one (4,3)

'seeing in the dark is often fun' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'one' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'for' acts as a link.

Can you help me to learn more?