Part of US welcomes Republican with extremely ghastly plan (8)
I believe the answer is:
strategy
'plan' is the definition.
(strategy is a kind of plan)
'part of us welcomes republican with extremely ghastly' is the wordplay.
'part of us' becomes 'state' ('state' is part of 'us'**).
'welcomes' means one lot of letters goes inside another (in sense of 'invites in').
'republican' becomes 'r' (abbreviation).
'with' says to put letters next to each other.
'extremely' means to remove the middle letters.
'ghastly' with its middle removed is 'gy'.
'state' placed around 'r' is 'strate'.
'strate'+'gy'='STRATEGY'
(Other definitions for strategy that I've seen before include "Plan of action, tactics" , "Scheme" , "Planning and direction of campaign" , "Grand design" , "Plan or policy to achieve something" .)