Crossword Genius

Grass up old lag, getting time inside as such? (10)

Ross

I believe the answer is:

shoplifter

I'm a little stuck... Click here to teach me more about this clue!

'time inside as such?' is the definition.
I can't tell whether this definition defines the answer.

'grass up old lag' is the wordplay.
'grass' becomes 'shop' (I've seen this before).
'up old lag' becomes 'lifter' (I can't explain this - if you can you should give a lot more credence to this answer).
'shop'+'lifter'='SHOPLIFTER'

'getting' is the link.

Can you help me to learn more?

(Other definitions for shoplifter that I've seen before include "guilty party" , "Thieving customer" , "A petty criminal" , "Criminal (in a retail context)" , "Robber" .)

I've seen this clue in The Times.
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