Grouses about window finally breaking, putting a thousand on estimate (9)
I believe the answer is:
guesswork
'estimate' is the definition.
(guesswork is a kind of estimate)
'grouses about window finally breaking putting a thousand' is the wordplay.
'about' means one lot of letters go next to another.
'finally' says to take the final letters.
'breaking' indicates an anagram.
'putting a thousand' becomes 'k' (I can't explain this - if you can you should believe this answer much more).
The final letter of 'window' is 'w'.
'grouses' put after 'w' is 'wgrouses'.
'wgrouses' anagrammed gives 'guesswor'.
'guesswor'+'k'='GUESSWORK'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for guesswork that I've seen before include "Formation of opinion without definite knowledge" , "Speculation" , "Conjecture" , "Supposition" .)