Twice contracting fungal growth in the arm (6)
I believe the answer is:
biceps
'in the arm' is the definition.
'twice contracting fungal growth' is the wordplay.
'twice' becomes 'bis' (I've seen this in other clues).
'contracting' indicates putting letters inside.
'fungal growth' becomes 'cep'.
'bis' enclosing 'cep' is 'BICEPS'.
(Other definitions for biceps that I've seen before include "Upper-arm muscle" , "Type of muscle" , "Contractor" , "Flexor muscle" , "Large upper arm muscle" .)