Forget to order the thing? (4)
I believe the answer is:
omit
'forget' is the definition.
(to omit to do something is to forget to do it)
'order the thing?' is the wordplay.
'order' becomes 'om' (Order of Merit).
'the thing?' becomes 'it'.
'om'+'it'='OMIT'
'to' is the link.
(Other definitions for omit that I've seen before include "Leave out or fail to do" , "Drop" , "Don't include" , "Disregard" , "Fail to mention" .)