If doily is moved behind small set (8)
I believe the answer is:
solidify
'set' is the definition.
(to solidify is to harden or set)
'if doily is moved behind small' is the wordplay.
'is moved' indicates anagramming the letters.
'behind' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'if'+'doily'='ifdoily'
'ifdoily' is an anagram of 'olidify'.
'olidify' put after 's' is 'SOLIDIFY'.
(Other definitions for solidify that I've seen before include "Harden" , "Cake" , "Become hard" .)