Become liable for Lions Club's regular rooftop meeting (5)
I believe the answer is:
incur
'become liable' is the definition.
(I've seen this in another clue)
'lions club's regular rooftop meeting' is the wordplay.
'regular' indicates alternate letters.
'rooftop' becomes 'r' (top of the word 'roof').
'meeting' says to put letters next to each other.
'lions'+'club'='lionsclub'
The alternate letters of 'lionsclub' are 'incu'.
'incu'+'r'='INCUR'
'for' acts as a link.
(Other definitions for incur that I've seen before include "Bring on oneself, probably unpleasant" , "Fall liable to" , "Bring on oneself, say costs" , "Become liable to (costs)" , "To become liable to, suffer" .)