Crossword Genius

My first girl - a growing one (6)

Ross

I believe the answer is:

myrtle

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

'a growing one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'my first girl' is the wordplay.
I cannot quite understand how this works, but
'my' is within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for myrtle that I've seen before include "Evergreen shrub with dark leaves and white flowers" , "Shrub; female name" , "Bush" , "Shrub with purple-black oval berries" , "Fragrant evergreen shrub" .)

I've seen this clue in the Evening Standard.
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