A safe boxing tactic's first to stun (7)
I believe the answer is:
astound
'stun' is the definition.
(thesaurus)
'a safe boxing tactic's first' is the wordplay.
'safe' becomes 'sound' ('sound' can be similar in meaning to 'safe'**).
'boxing' indicates putting letters inside ('box' can mean contain, as in to put in a box).
'first' indicates taking the first letters.
The first letter of 'tactics' is 't'.
'sound' going around 't' is 'stound'.
'a'+'stound'='ASTOUND'
'to' acts as a link.
(Other definitions for astound that I've seen before include "Startle" , "Stupefy" , "Bewilder" , "Flabbergast" , "Shock, surprise greatly" .)