The French returning with irregular forces in the open air (8)
I believe the answer is:
alfresco
'in the open air' is the definition.
(I've seen this before)
'the french returning with irregular forces' is the wordplay.
'the french' becomes 'la' ('the' in French).
'returning' says the letters should be written backwards.
'with' says to put letters next to each other.
'irregular' indicates anagramming the letters.
'la' reversed gives 'al'.
'forces' with letters rearranged gives 'fresco'.
'al'+'fresco'='ALFRESCO'
(Other definitions for alfresco that I've seen before include "where it's cool" , "Out of doors" , "Outside" , "Outdoors, meal perhaps" , "Located outdoors" .)