Demonstrate range under pressure (5)
I believe the answer is:
prove
'demonstrate' is the definition.
(I know that demonstrate can be written as prove)
'range under pressure' is the wordplay.
'range' becomes 'rove' (synonyms).
'under' means one lot of letters go next to another (below, in a down clue).
'pressure' becomes 'p' (scientific abbreviation).
'rove' put after 'p' is 'PROVE'.
(Other definitions for prove that I've seen before include "Ascertain" , "Demonstrate by evidence" , "Show to be true" , "Verify" , "Demonstrate beyond doubt" .)