Extremely testy on phone, mostly about old storm (7)
I believe the answer is:
typhoon
'storm' is the definition.
(I know that typhoon is a type of cyclone)
'extremely testy on phone mostly about old' is the wordplay.
'extremely' means to remove the middle letters (letters on the extremes of the word are used).
'on' means one lot of letters go next to another.
'mostly' means to remove the last letter (most but not all of the word).
'about' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'testy' with its centre removed is 'ty'.
'phone' with its last letter taken away is 'phon'.
'phon' going around 'o' is 'phoon'.
'ty'+'phoon'='TYPHOON'
(Other definitions for typhoon that I've seen before include "Violent cyclonic storm" , "Indian Ocean tropical storm" , "Tropical cyclone" , "Cyclonic sea storm" , "Indian Ocean cyclone" .)