Contemporary people mostly reserved about Republican nationalist (7)
I believe the answer is:
moderns
'contemporary people' is the definition.
(forward-thinking or modern people)
'mostly reserved about republican nationalist' is the wordplay.
'mostly' means to remove the last letter.
'reserved' becomes 'modest' (I've seen this before).
'about' is an insertion indicator.
'republican' becomes 'r' (abbreviation).
'nationalist' becomes 'n' (abbreviation in politics).
'modest' with its final letter taken off is 'modes'.
'r'+'n'='rn'
'modes' going around 'rn' is 'MODERNS'.
(Another definition for moderns that I've seen is " People today".)