We hear this person shows happiness in a glance (3-4)

'a glance' is the definition.
(historical term for a glance from the eye)

'we hear this person shows happiness' is the wordplay.
'we hear' indicates a 'sounds like' (homophone) clue.
'this person' becomes 'I' (the person writing).
'shows happiness' becomes 'beam' (I can't justify this - if you can you should give a lot more credence to this answer).
'i' is a homophone of 'eye'.
'eye'+'beam'='EYE-BEAM'

'in' is the link.