Pay attention to short letter (4)
I believe the answer is:
note
This clue is a double definition.
'pay attention to' is the first definition.
(thesaurus)
'short letter' is the second definition.
(synonyms)
(Other definitions for note that I've seen before include "Brief letter - distinction" , "held by piano teacher" , "Be mindful of" , "Observe; short letter" , "Sound, record" .)