What may be an essential for campers proves it -- sum involved? (6,5)
I believe the answer is:
primus stove
'what may be an essential for campers' is the definition.
(type of stove used when camping)
'proves it sum involved?' is the wordplay.
'involved?' is an anagram indicator (involved can mean confusing or complex).
'proves'+'it'+'sum'='provesitsum'
'provesitsum' is an anagram of 'PRIMUS STOVE'.
(Other definitions for primus stove that I've seen before include "Portable cooker" , "camping item" .)