High clouds: I start getting perturbed after a lot at sea (10)

I believe the answer is:

altostrati

I'm a little stuck... Click here to teach me more about this clue!

'high clouds' is the definition.
'altostrati' can be an answer for 'clouds' (I've seen this before). I am not sure about the 'high' bit.

'i start getting perturbed after a lot at sea' is the wordplay.
'getting perturbed' is an anagram indicator.
'after' means one lot of letters go next to another.
'at sea' is an anagram indicator.
'i'+'start'='istart'
'istart' with letters rearranged gives 'strati'.
'a'+'lot'='alot'
'alot' is an anagram of 'alto'.
'strati' put after 'alto' is 'ALTOSTRATI'.

Can you help me to learn more?

(Other definitions for altostrati that I've seen before include "clouds" , "Layers of cloud" .)

I've seen this clue in The Independent.

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