Sympathetic to sides generous to Newcastle (6)
I believe the answer is:
gentle
'sympathetic' is the definition.
(thesaurus)
'sides generous to newcastle' is the wordplay.
'sides' means to remove the middle letters (I've seen 'sides of' mean this (only the letters on the sides remain)).
'generous' becomes 'large' ('large' can be similar in meaning to 'generous').
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'newcastle' becomes 'gent' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'large' with its middle taken out is 'le'.
'le' put after 'gent' is 'GENTLE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for gentle that I've seen before include "that may help angler catch fish" , "Soft and mild, quiet and soothing" , "Soothing - light" , "'Do not go ... into that good night'/Thomas" , "Mild, kind, tender" .)