Feet — setter’s two to start with (5)
I believe the answer is:
iambi
'feet' is the definition.
(plural of iamb, a type of metrical foot in poetry)
'setter's two to start with' is the wordplay.
'setter' becomes 'I' (the person who set this crossword).
'two' becomes 'iamb' (I can't justify this - if you can you should give a lot more credence to this answer).
'to start with' says to put letters next to each other.
'i' put after 'iamb' is 'IAMBI'.
Can you help me to learn more?
(Other definitions for iambi that I've seen before include "Film theatre" , "Poetic feet" , "Poetic pieces" , "Metrical patterns" , "Concierto de Aranjuez composer" .)