Kent area, with unity of purpose, mostly in time (6)
I believe the answer is:
season
'time' is the definition.
(I've seen this before)
'kent area with unity of purpose mostly' is the wordplay.
'kent area' becomes 'se'.
'with unity of purpose' becomes 'asone' (as in acting 'as one').
'mostly' means to remove the last letter.
'asone' with its last letter taken off is 'ason'.
'se'+'ason'='SEASON'
'in' acts as a link.
(Other definitions for season that I've seen before include "such as 'Summertime'" , "Particular time (for activity, conditions etc)" , "Major division of year characterised by weather and growth" , "Add flavour at a certain time of year" , "temper" .)