Intensify struggle over stupid person abandoning pet (4,2)
I believe the answer is:
warm up
'intensify' is the definition.
'struggle over stupid person abandoning pet' is the wordplay.
'struggle over' becomes 'war' (warring is a kind of struggling. I am not sure about the 'over' bit.).
'stupid person' becomes 'muppet'.
'abandoning' indicates named letters should be taken away.
'muppet' with 'pet' removed is 'mup'.
'war'+'mup'='WARM-UP'
(Other definitions for warm up that I've seen before include "race preparation" , "Exercise before race - reheat food - become more animated" , "Exercises needed before running" , "Rehearse" , "Prepare for" .)