Sign record or mag unexpectedly (8)
I believe the answer is:
logogram
'sign' is the definition.
(logogram is a kind of sign)
'record or mag unexpectedly' is the wordplay.
'record' becomes 'log' (logging is a kind of recording).
'unexpectedly' indicates an anagram.
'or'+'mag'='ormag'
'ormag' is an anagram of 'ogram'.
'log'+'ogram'='LOGOGRAM'
(Other definitions for logogram that I've seen before include "Cryptic clue, eg, =" , "symbol" , "A single sign for a word" .)