Time to slip up -- I attempt to cross old field (9)
I believe the answer is:
territory
'field' is the definition.
(thesaurus)
'time to slip up i attempt to cross old' is the wordplay.
'time' becomes 't'.
'to slip up' becomes 'err' (I've seen this before).
'attempt' becomes 'try' (I've seen this before).
'to cross' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'try' placed around 'o' is 'tory'.
't'+'err'+'i'+'tory'='TERRITORY'
(Other definitions for territory that I've seen before include "Tract of land" , "Area under the jurisdiction of a sovereign state" , "Geographical region" , "ground" , "Constituency" .)