When one's getting on with female penning graffito (6)
I believe the answer is:
dotage
'when one's getting on' is the definition.
(period when one ages or 'gets on')
'female penning graffito' is the wordplay.
'female' becomes 'doe' (I've seen this before).
'penning' means one lot of letters goes inside another.
'graffito' becomes 'tag' (a tag is a name added to graffiti).
'doe' going around 'tag' is 'DOTAGE'.
'with' acts as a link.
(Other definitions for dotage that I've seen before include "when any activity may become difficult?" , "decrepitude" , "Senile stage" , "Old, weak period of life" , "Old age, senility" .)