Not fat, I am tucking in, finding nutrient (7)
I believe the answer is:
thiamin
'nutrient' is the definition.
(thiamin is a kind of nutrient)
'not fat i am tucking in' is the wordplay.
'not fat' becomes 'thin' ('thin' is the opposite to 'fat').
'tucking in' means one lot of letters goes inside another.
'i'+'am'='iam'
'thin' enclosing 'iam' is 'THIAMIN'.
'finding' acts as a link.
(Other definitions for thiamin that I've seen before include "Vitamin B 1" , "food ingredient" , "Vitamin in cereals, beans, etc" , "vitamin required" , "Essential nutrition" .)