Fishes for a pound a second, clutching heart! (9)

'fishes' is the definition.
(albacore is a kind of fish)

'a pound a second clutching heart' is the wordplay.
I cannot quite understand how this works, but
'a' is found in the answer.
'pound' could be 'lb' ('lb' can be a synonym of 'pound') and 'lb' is present in the answer.
'second' could be 's' (common abbreviation) and 's' is found in the answer.
'heart' could be 'core' ('core' can be a synonym of 'heart') and 'core' is located in the answer.
A single letter 'a' remains which might be clued in a way I don't see.
This explanation may well be incorrect...

'for' is the link.

Can you help me to learn more?