Gradient of playing field (5)
I believe the answer is:
pitch
I believe this is a double definition.
'gradient' is the first definition.
(thesaurus)
'playing field' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for pitch that I've seen before include "Tarry stuff for football field?" , "Tone - football field" , "Set up a tent" , "Football ground" , "Playing field as for football" .)