Every so often trips by girl matter (6)
I believe the answer is:
tissue
'matter' is the definition.
(tissue is a kind of matter)
'every so often trips by girl' is the wordplay.
'every so often' indicates alternate letters.
'by' is a charade indicator (letters next to each other).
'girl' becomes 'sue' (girl's name).
The alternate letters of 'trips' are 'tis'.
'tis'+'sue'='TISSUE'
(Other definitions for tissue that I've seen before include "Cells sharing a function" , "See 11" , "Thin stuff" , "cellular network" , "Delicate fabric" .)