Support dispatch commander put in (6)
I believe the answer is:
second
'support' is the definition.
(seconding is a kind of supporting)
'dispatch commander put in' is the wordplay.
'dispatch' becomes 'send' (I've seen this before).
'commander' becomes 'co'.
'put in' means one lot of letters goes inside another.
'send' going around 'co' is 'SECOND'.
(Other definitions for second that I've seen before include "Support - a little time" , "Support, assist" , "Further opportunity" , "jiffy" , "Basic unit of time" .)