Frank's happy to start firing a gun (4,2)
I believe the answer is:
open up
'start firing a gun' is the definition.
I can't judge whether this defines the answer.
'frank's happy' is the wordplay.
'frank' becomes 'open' (synonyms).
'happy' becomes 'up' (I've seen this in other clues).
'open'+'up'='OPEN UP'
'to' is the link.
Can you help me to learn more?
(Other definitions for open up that I've seen before include "Speak freely and without restraint" , "Speak freely or unlock door" , "speak his heart" , "Begin to take shots" , "become less cloistered" .)