Encourage bond in yellow part of ovum (3,4)
I believe the answer is:
egg yolk
'yellow part of ovum' is the definition.
'egg yolk' can be an answer for 'yellow' (I've seen this in another clue). I am not certain of the 'part of ovum' bit.
'encourage bond' is the wordplay.
'encourage' becomes 'egg'.
'bond' becomes 'yolk' (I can't justify this - if you can you should believe this answer much more).
'egg'+'yolk'='EGG YOLK'
'in' is the link.
Can you help me to learn more?
(Other definitions for egg yolk that I've seen before include "Part of an ovum" , "Omelette ingredient" , "17 [CAKE] ingredient" , "What soldiers are dipped into" .)