I'd boil uncontrollably in desire (6)
I believe the answer is:
libido
'desire' is the definition.
(libido is a kind of desire)
'i'd boil uncontrollably in' is the wordplay.
'uncontrollably' indicates an anagram (I've seen this in other clues).
'in' indicates putting letters inside.
'boil' anagrammed gives 'libo'.
'id' put inside 'libo' is 'LIBIDO'.
(Other definitions for libido that I've seen before include "Instinctive desire" , "Sexual impulse" , "Strong urge" , "I'd boil over with sexual energy" , "Emotional energy associated with sexual desire" .)