Replace spades, having finished with machinery (8)
I believe the answer is:
supplant
'replace' is the definition.
(I know that replace can be written as supplant)
'spades having finished with machinery' is the wordplay.
'spades' becomes 's' (abbreviation in card games).
'having' says to put letters next to each other.
'finished' becomes 'up' (eg 'time's up' means the time is finished).
'with' is a charade indicator (letters next to each other).
'machinery' becomes 'plant' (I've seen this before).
's'+'up'+'plant'='SUPPLANT'
(Other definitions for supplant that I've seen before include "take another's place" , "Supersede and replace" , "Oust" , "Displace" .)