Working diligently is difficult for a diminutive flier (4,2,2)
I believe the answer is:
hard at it
'working diligently' is the definition.
(I've seen this in another clue)
'difficult for a diminutive flier' is the wordplay.
'difficult' becomes 'hard' (synonyms).
'for' says to put letters next to each other.
'diminutive flier' becomes 'tit' (type of bird. I am not sure about the 'diminutive' bit.).
'hard'+'a'+'tit'='HARD AT IT'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for hard at it that I've seen before include "Very busy - that raid (anag)" , "Really busy" , "Working very busily" .)