Single call to arms not entirely hard to deliver? (7)
I believe the answer is:
onerous
'hard to deliver?' is the definition.
'single call to arms not entirely' is the wordplay.
'single' becomes 'one' (I've seen this before**).
'call to arms' becomes 'rouse'.
'not entirely' means to remove the last letter.
'rouse' with its final letter removed is 'rous'.
'one'+'rous'='ONEROUS'
(Other definitions for onerous that I've seen before include "Toilsome" , "Burdensome, taxing" , "No euros? That's burdensome" , "Burdensome, oppressive" , "Effortful" .)