Small egg on big roll (5)
I believe the answer is:
surge
'big roll' is the definition.
'surge' can be an answer for 'roll' (I've seen this before). I'm not certain of the 'big' bit.
'small egg' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'egg' becomes 'urge' (I've seen this before).
's'+'urge'='SURGE'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for surge that I've seen before include "Sudden increase in power" , "Sudden large increase" , "Rise rapidly" , "Rise suddenly" , "Sudden push forward" .)