Weekly or monthly, perhaps, obtained larva (6)
I believe the answer is:
maggot
'larva' is the definition.
(I know that maggot is a type of grub)
'weekly or monthly perhaps obtained' is the wordplay.
'weekly or monthly perhaps' becomes 'mag' (weekly or monthly magazine).
'obtained' becomes 'got' ('get' can be a synonym of 'obtain').
'mag'+'got'='MAGGOT'
(Other definitions for maggot that I've seen before include "youngster to fly" , "immature flyer" , "Legless grub" , "pest" , "Lava of a fly" .)