Little by little, guard irritated colleague (9)
I believe the answer is:
gradually
'little by little' is the definition.
(I've seen this before)
'guard irritated colleague' is the wordplay.
'irritated' indicates anagramming the letters (letters disturbed or irritated).
'colleague' becomes 'ally' (ally can mean a colleague or associate).
'guard' is an anagram of 'gradu'.
'gradu'+'ally'='GRADUALLY'
(Other definitions for gradually that I've seen before include "Bit by bit" , "slow on the uptake?" , "Developing slowly and by degrees" , "A little at a time - slowly" , "over a period?" .)