Shooting some mosquitoes, by the sound of it (5)
I believe the answer is:
skeet
'shooting some mosquitoes' is the definition.
'skeet' can be an answer for 'shooting' (US shooting sport). I am not sure about the 'some mosquitoes' bit.
'by the sound of it' is the wordplay.
'by' says to put letters next to each other.
'the sound of' becomes 'skee' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'skee' next to 't' is 'SKEET'.
Can you help me to learn more?
(Other definitions for skeet that I've seen before include "A form of clay-pigeon shooting" , "American clay pigeon shooting" , "Form of clay pigeon shooting" , "US version of a shooting sport involving clay pigeons" .)