Plane perhaps crossing a large Irish town (6)
I believe the answer is:
tralee
'irish town' is the definition.
(I've seen this in another clue)
'plane perhaps crossing a large' is the wordplay.
'plane perhaps' becomes 'tree' (I've seen this before).
'crossing' is an insertion indicator (to cross can mean to go over or around).
'large' becomes 'L' (eg in clothes sizes).
'a'+'l'='al'
'tree' enclosing 'al' is 'TRALEE'.
(Other definitions for tralee that I've seen before include "'The Rose of ------': song/film title featuring a town in County Kerry, Ireland" , "Kerry town of Roses" .)