Increase in work for the summer (8)
I believe the answer is:
addition
'the summer' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'increase in work' is the wordplay.
'increase' becomes 'add' (I've seen this in another clue).
'in work' becomes 'ition' (I can't justify this - if you can you should believe this answer much more).
'add'+'ition'='ADDITION'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for addition that I've seen before include "Combination of numbers to find the sum" , "Mathematical procedure" , "an arithmetical process" , "Tally, tot" , "plus" .)