Upstanding girl holding one fibre (5)
I believe the answer is:
sisal
'one fibre' is the definition.
(sisal is a kind of fibre)
'upstanding girl holding' is the wordplay.
'upstanding' becomes 'is' (I can't explain this - if you can you should give a lot more credence to this answer).
'girl' becomes 'sal' (feminine given name).
'holding' means one lot of letters goes inside another.
'is' placed within 'sal' is 'SISAL'.
Can you help me to learn more?
(Other definitions for sisal that I've seen before include "Fibre-yielding plant" , "Agave rope-fibre" , "Agave cultivated for its fibre" , "Strong rope fibre" , "Hemp fibre" .)