Assess fruit across Ukraine's capital (5)
I believe the answer is:
gauge
'assess' is the definition.
(thesaurus)
'fruit across ukraine's capital' is the wordplay.
'fruit' becomes 'gage' (a greengage or gage is a type of fruit).
'across' is an insertion indicator.
'capital' indicates taking the first letters.
The first letter of 'ukraine' is 'u'.
'gage' placed around 'u' is 'GAUGE'.
(Other definitions for gauge that I've seen before include "standard size" , "Distance between the rails of a railway track" , "Meter" , "Measuring instrument" , "Measure - estimate" .)