Reasonable manner and pace (8)
I believe the answer is:
moderate
'reasonable' is the definition.
(I've seen this before)
'manner and pace' is the wordplay.
'manner' becomes 'mode' ('mode' can be a synonym of 'manner'**).
'and' means one lot of letters go next to another.
'pace' becomes 'rate' ('rate' can be a synonym of 'pace').
'mode'+'rate'='MODERATE'
(Other definitions for moderate that I've seen before include "Temper" , "Average in amount" , "Average, middle rate" , "Unextreme" , "Not extreme - not very good" .)