An offhand impression (11)
I believe the answer is:
fingerprint
'an offhand impression' is the definition.
'fingerprint' can be an answer for 'impression' (thesaurus). I am not sure about the 'an offhand' bit.
'offhand impression' is the wordplay.
'offhand' becomes 'finger' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'impression' becomes 'print' (impression created by a printer).
'finger'+'print'='FINGERPRINT'
Can you help me to learn more?
(Other definitions for fingerprint that I've seen before include "Mark left by handler" , "Dab" , "means of identification" .)