Formula for getting fat — I lose it (5,1,3)
I believe the answer is:
throw a fit
'formula for getting fat i lose it' is the definition.
I can't judge whether this definition defines the answer.
'getting fat i lose it' is the wordplay.
'getting' is a charade indicator (letters next to each other).
'lose' is an anagram indicator (I've seen 'lost' mean this).
'it' becomes 'throw' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fat'+'i'='fati'
'fati' is an anagram of 'afit'.
'afit' after 'throw' is 'THROW A FIT'.
(Other definitions for throw a fit that I've seen before include "Rage" , "Behave wildly" , "get very angry" , "Go mad" , "Express extreme anger" .)