Poles securing trained part of plant (6)
I believe the answer is:
stamen
'part of plant' is the definition.
(fertilising organ of a flower)
'poles securing trained' is the wordplay.
'poles' becomes 'sn' (North and South poles).
'securing' means one lot of letters goes inside another.
'trained' becomes 'tame' (I can't explain this - if you can you should give a lot more credence to this answer).
'sn' enclosing 'tame' is 'STAMEN'.
Can you help me to learn more?
(Other definitions for stamen that I've seen before include "Male ditto" , "Pollen-bearing organ of flowering plant" , "Flower's male reproductive organ" , "Pollen-bearing part of a flower" , "Male part of flower" .)