Additional part of complex training programme (5)
I believe the answer is:
extra
'additional' is the definition.
(I know that additional can be written as extra)
'part of complex training programme' is the wordplay.
'part of' says the answer is hidden in the clue.
'EXTRA' is hidden within 'complex training programme'.
(Other definitions for extra that I've seen before include "special edition of the paper" , "Minor actor in crowd scene" , "additional supplies" , "Especially" , "Bit-part actor" .)