Cycling sailor is (I reckon) very high (9)

I believe the answer is:
altissimo
'very high' is the definition.
(musical term)
'cycling sailor is i reckon' is the wordplay.
I cannot quite see how this works, but
'is' is found in the answer.
'i' could be 'a' and 'a' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for altissimo that I've seen before include "Very highly placed" , "Very high musical pitch" , "Very highly pitched" , "one is almost dizzy" .)
