Assistant having a plan snubbed (4)
I believe the answer is:
aide
'assistant' is the definition.
(aide is a kind of assistant)
'a plan snubbed' is the wordplay.
'plan' becomes 'idea' (I've seen this before).
'snubbed' means to remove the last letter (could historically mean cutting the end off).
'idea' with its final letter taken away is 'ide'.
'a'+'ide'='AIDE'
'having' acts as a link.
(Other definitions for aide that I've seen before include "Adviser or assistant" , "Officer's assistant" , "Right-hand man/woman" , "Confidential assistant or advisor" , "Officer in attendance" .)