Economical inclusion of key in opening part of an Italian composition (7)
I believe the answer is:
sparing
'economical' is the definition.
(I know that economical can be written as sparing)
'inclusion of key in opening part of an italian composition' is the wordplay.
'inclusion of key' becomes 'a' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'opening part of an italian composition' becomes 'spring' (as in Vivaldi's Four Seasons).
'a' placed into 'spring' is 'SPARING'.
(Other definitions for sparing that I've seen before include "Moderate, economical" , "Thrifty" , "Economic (with resources)" , "Economical; having mercy" , "mean" .)