Do something with current to produce muscle protein (5)
I believe the answer is:
actin
'muscle protein' is the definition.
(I've seen this in another clue)
'do something with current' is the wordplay.
'do something' becomes 'act' (to act is to do something).
'with' means one lot of letters go next to another.
'current' becomes 'in' ('in' can mean current or trendy).
'act'+'in'='ACTIN'
'to produce' acts as a link.
(Other definitions for actin that I've seen before include "protein" , "What can affect muscles" .)