Mariner's first two letters about heavenly fish gave them some diversion (8)
I believe the answer is:
escapism
'some diversion' is the definition.
(escapism is a kind of diversion)
'mariner's first two letters about heavenly fish gave them' is the wordplay.
I cannot really see how this works, but
'first' could be 'i' (Roman numeral for one or first) and 'i' is found in the answer.
'letters' could be 'es' (more than one E) and 'es' is found within the answer.
'about' could be 'ca' (abbreviation for circa) and 'ca' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for escapism that I've seen before include "Tendency to flee reality" , "Sim space (anag)" , "Avoiding life's realities by indulging in fantasies" , "Tendency to avoid reality" , "Inclination to retreat from reality" .)