Girl on time could be put in the picture (7)
I believe the answer is:
tessera
'be put in the picture' is the definition.
The definition suggests a singular noun which matches the answer.
'girl on time' is the wordplay.
'girl' becomes 'tess' (girl's name).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'time' becomes 'era' (an era is a period of time).
'tess'+'era'='TESSERA'
'could' is the link.
Can you help me to learn more?
(Other definitions for tessera that I've seen before include "part of a mosaic" , "Mosaic stone" , "Bit of design" , "Die" , "Small mosaic piece" .)