Prepare to obtain prompt (3,5)
I believe the answer is:
get ready
'prepare' is the definition.
(I've seen this before)
'obtain prompt' is the wordplay.
'obtain' becomes 'get' ('get' can be a synonym of 'obtain').
'prompt' becomes 'ready' ('ready' can be similar in meaning to 'prompt').
'get'+'ready'='GET READY'
'to' acts as a link.
(Other definitions for get ready that I've seen before include "Prepare oneself" , "third of three" .)