They make the best of the chance of a lifetime going up river to harbours around African capital (12)

'they make the best of the chance of a lifetime going up river' is the definition.
The definition suggests an adverb but the answer is not.

'harbours around african capital' is the wordplay.
I cannot quite understand how this works, but
'around' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'capital' could be 'tunis' (capital of Tunisia) and 'tunis' is located in the answer.
'harbours' could be 'ports' (I've seen this before) and 'ports' is located in the remaining letters.
A single letter 'p' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

'to' acts as a link.

Can you help me to learn more?