Long to take on board science of fish (7)
I believe the answer is:
piscine
'of fish' is the definition.
(I know this)
'long to take on board science' is the wordplay.
'long' becomes 'pine' (to pine for someone is to long for them).
'to take on board' is an insertion indicator.
'science' becomes 'sci' (eg in 'poli sci' for political science).
'pine' enclosing 'sci' is 'PISCINE'.
(Other definitions for piscine that I've seen before include "Resembling a fish" , "about aquatic life" .)