Three men regularly seen in plane perhaps (4)
I believe the answer is:
tree
'plane perhaps' is the definition.
(I've seen this before)
'three men regularly seen' is the wordplay.
'regularly seen' indicates alternate letters.
'three'+'men'='threemen'
The alternating letters of 'threemen' are 'TREE'.
'in' is the link.
(Other definitions for tree that I've seen before include "Large plant with trunk" , "Plant with bark" , "Large woody plant (4)" , "Sequoia, for example" , "Christmas decoration" .)