Unite king and fool (4)
I believe the answer is:
knit
'unite' is the definition.
(I've seen this before)
'king and fool' is the wordplay.
'king' becomes 'k'.
'and' means one lot of letters go next to another.
'fool' becomes 'nit' (I've seen this before).
'k'+'nit'='KNIT'
(Other definitions for knit that I've seen before include "Make textiles with needles and wool" , "(Bone) heal; contract (brows)" , "Make a scarf, say" , "Link closely together" , "Draw (brows) together" .)