Drop by to watch TV (4,2)
I believe the answer is:
look in
'drop' is the definition.
(thesaurus)
'by to watch tv' is the wordplay.
'by' says to put letters next to each other.
'to watch' becomes 'look' (I've seen this in another clue).
'tv' becomes 'in' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'look' put next to 'in' is 'LOOK-IN'.
Can you help me to learn more?
(Other definitions for look in that I've seen before include "Make a short visit" , "Chance to participate" .)