Serious attention given to collection of machine guns (7)
I believe the answer is:
earnest
'serious' is the definition.
(similar in meaning)
'attention given to collection of machine guns' is the wordplay.
'attention' becomes 'ear' (having someone's ear is having their attention).
'given to' means one lot of letters go next to another.
'collection of machine guns' becomes 'nest' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ear'+'nest'='EARNEST'
Can you help me to learn more?
(Other definitions for earnest that I've seen before include "See 1" , "Seriously eager" , "serious attention" , "Sincere; enthusiastic" , "Pledge - far from frivolous" .)