Drop in to order a second (8)
I believe the answer is:
decrease
'drop' is the definition.
(thesaurus)
'in to order a second' is the wordplay.
'in' indicates putting letters inside.
'to order' becomes 'decree' ('decree' can be a synonym of 'order').
'a second' becomes 'as' (I've seen this in other clues).
'decree' going around 'as' is 'DECREASE'.
(Other definitions for decrease that I've seen before include "Diminish, lessen" , "Turn down" , "Reduce in size" , "Make or become less" , "subsidence" .)