Figures showing increase has desired effect (8)
I believe the answer is:
waxworks
'figures' is the definition.
(waxwork figures)
'increase has desired effect' is the wordplay.
'increase' becomes 'wax' (waxing is a kind of increasing).
'has desired effect' becomes 'works' (synonyms).
'wax'+'works'='WAXWORKS'
'showing' is the link.
(Other definitions for waxworks that I've seen before include "Figures, eg in Warwick Castle" , "Madame Tussaud's eg" , "Exhibition of modelled figures" , "Exhibition of effigies" , "Dummies" .)