Within reason, limit friend to fair share (10)

I believe the answer is:

rationally

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'within reason' is the definition.
(thesaurus)

'friend to fair share' is the wordplay.
'friend' becomes 'ally' (an ally is a friend or supporter).
'to' means one lot of letters go next to another (I've seen this in other clues).
'fair share' becomes 'ration' (a ration is a share of a limited resource. I am not sure about the 'fair' bit.).
'ally' put after 'ration' is 'RATIONALLY'.

'limit' acts as a link.
This may not be right. It may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for rationally that I've seen before include "in sensible fashion" , "in a way that's sensible" , "By logical means" .)

I've seen this clue in The Irish Independent.

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