Fish moving up a southern coastline (8)
I believe the answer is:
seashore
'a southern coastline' is the definition.
'seashore' can be an answer for 'coastline' (thesaurus). I'm unsure of the 'a southern' bit.
'fish moving up' is the wordplay.
'fish' becomes 'sea fish' (sea fish is a kind of fish).
'moving up' becomes 'shore' (I can't explain this - if you can you should give a lot more credence to this answer).
'sea'+'shore'='SEASHORE'
Can you help me to learn more?
(Other definitions for seashore that I've seen before include "Coast, of an ocean say" , "Nautical cry" , "Coast, stand" , "Littoral" , "Strand, beach" .)