Additional feature of complex traits (5)
I believe the answer is:
extra
'additional' is the definition.
(extra means additional or surplus)
'feature of complex traits' is the wordplay.
'feature of' indicates a hidden word.
'EXTRA' is hidden within 'complex traits'.
(Other definitions for extra that I've seen before include "Surcharge" , "(Theatrical) spear carrier" , "Additional, like non-speaking actor" , "additional supplies" , "not included" .)