Become aware of things smelt, heard but not seen (5)
I believe the answer is:
sense
'become aware of' is the definition.
(I've seen this in another clue)
'things smelt heard but not seen' is the wordplay.
'things smelt' becomes 'scents' (scents can be smelled).
'heard but not seen' shows a homophone (sound like).
'scents' is a homophone of 'SENSE'.
(Other definitions for sense that I've seen before include "Feeling - reason" , "Awareness" , "smell, for example" , "Perceptual faculty" , "Faculty of perception" .)