In short, for once is not right on key (7)
I believe the answer is:
concise
'in short' is the definition.
'concise' can be an answer for 'short' (similar in meaning). I am not sure about the 'in' bit.
'once is not right on key' is the wordplay.
'not right' indicates an anagram.
'on' is an insertion indicator (as in clothing 'on' a person).
'key' becomes 'c' (musical key).
'once'+'is'='onceis'
'onceis' is an anagram of 'oncise'.
'oncise' going around 'c' is 'CONCISE'.
'for' is the link.
Can you help me to learn more?
(Other definitions for concise that I've seen before include "Brief and to the point" , "Succinct" , "Brief but pertinent" , "Expressing much in few words" , "Brief but comprehensive" .)