Sheepish female at heart, but a gem! (5)

I believe the answer is:

jewel

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'a gem' is the definition.
('jewel' can be a synonym of 'gem')

'sheepish female at heart' is the wordplay.
'sheepish' becomes 'jew' (I can't justify this - if you can you should believe this answer much more).
'female' becomes 'thelma'.
'at heart' says to take the centre.
The middle of 'thelma' is 'el'.
'jew'+'el'='JEWEL'

'but' is the link.

Can you help me to learn more?

(Other definitions for jewel that I've seen before include "Gemstone" , "Precious stone or mineral" , "eg Ruby" , "(Semi) precious stone" , "The - That Was Ours (Dexter)" .)

I've seen this clue in the Evening Standard.

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