Clue: What shifts fat? Tight clothes and active role (11)
Answer: CHOLESTEROL
Explanation: What shifts fat? Tight clothes and active role (11)
***(clothes)** [anag:*tight*] + ***(role)** [anag:*active*] 
*Not sure what the definition is supposed to be here. Cholesterol is a type of fat, and does not shift fat, as far as my non-scientific thinking goes, but happy to be corrected if I have got it wrong.