Republican must be in however unfashionable everywhere (10)
I believe the answer is:
throughout
'everywhere' is the definition.
(I've seen this before)
'republican must be in however unfashionable' is the wordplay.
'republican' becomes 'r' (abbreviation).
'must be in' means one lot of letters goes inside another.
'however' becomes 'though' (synonyms).
'unfashionable' becomes 'out' (similar in meaning).
'r' going within 'though' is 'through'.
'through'+'out'='THROUGHOUT'
(Other definitions for throughout that I've seen before include "Completely" , "from start to finish" , "during the entire proceedings" , "Everywhere" , "all the time" .)