Took the part of another (5)
I believe the answer is:
sided
'took' is the definition.
(siding is a kind of taking)
'part of another' is the wordplay.
'part' becomes 'si' (si is a kind of part).
'of another' becomes 'ded' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'si'+'ded'='SIDED'
'the' is the link.
Can you help me to learn more?
(Other definitions for sided that I've seen before include "Allied oneself (with one party)" , "Took part with person in a dispute" , "Showed solidarity (with)" .)