Greeting the start of summer on a musical instrument (6)
I believe the answer is:
salute
'greeting' is the definition.
(salute is a kind of greeting)
'the start of summer on a musical instrument' is the wordplay.
'the start of' says to take the initial letters.
'on' is a charade indicator (letters next to each other).
'musical instrument' becomes 'lute' (lute is a kind of musical instrument**).
The first letter of 'summer' is 's'.
's'+'a'+'lute'='SALUTE'
(Other definitions for salute that I've seen before include "Bring hand smartly to cap" , "A greeting, often military" , "Kind of welcome" , "Mark" , "Greet in a friendly way" .)