Exercising power every twelve months, briefly becoming irritable (7)
I believe the answer is:
peppery
'irritable' is the definition.
(I've seen this before)
'exercising power every twelve months briefly' is the wordplay.
'exercising' becomes 'pe' (abbreviation for Physical Education).
'power' becomes 'p' (symbol in physics).
'every' becomes 'per' ('per' means 'for each' or 'for every').
'twelve months' becomes 'yr' (yr is abbreviation for year).
'briefly' means to remove the last letter.
'yr' with its final letter removed is 'y'.
'pe'+'p'+'per'+'y'='PEPPERY'
'becoming' acts as a link.
(Other definitions for peppery that I've seen before include "Highly spiced" , "Irritable" , "Very spicy" , "Quick-tempered" .)