Attack with a jack, say (7)
I believe the answer is:
assault
'attack' is the definition.
(synonyms)
'with a jack say' is the wordplay.
'with' is an insertion indicator.
'jack' becomes 'ass' (I've seen this before).
'say' becomes 'ult' (I can't explain this - if you can you should believe this answer much more).
'ass'+'ult'='assult'
'a' placed inside 'assult' is 'ASSAULT'.
Can you help me to learn more?
(Other definitions for assault that I've seen before include "The fool starts the attack" , "Attack physically" , "Attack violently" , "raid" , "Launch an attack on" .)