Frequently on about fort, European (5)
I believe the answer is:
often
'frequently' is the definition.
(synonyms)
'on about fort european' is the wordplay.
'about' is an insertion indicator.
'fort' becomes 'ft' (abbreviation in place names).
'european' becomes 'E' (abbreviation e.g. EU).
'ft'+'e'='fte'
'on' going around 'fte' is 'OFTEN'.
(Other definitions for often that I've seen before include "Frequently, at short intervals" , "A great deal" , "In many instances" , "Frequently, not seldom" , "Repeatedly, many times" .)