The fundamental reasons for the fixed amount with beers (10)

'the fundamental reasons' is the definition.
'rationales' can be an answer for 'reasons' (I've seen this in another clue). I'm not certain of the 'the fundamental' bit.

'fixed amount with beers' is the wordplay.
'fixed amount' becomes 'ration' (I have seen 'adequate amount' mean 'ration' so perhaps 'amount' could also mean 'ration'. I am not sure about the 'fixed' bit.).
'with' says to put letters next to each other.
'beers' becomes 'ales' (I've seen this before).
'ration'+'ales'='RATIONALES'

'for the' is the link.

Can you help me to learn more?