Occasionally fair legal department (4)
I believe the answer is:
area
'department' is the definition.
(someone's department is their area of expertise)
'occasionally fair legal' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'fair'+'legal'='fairlegal'
The alternating letters of 'fairlegal' are 'AREA'.
(Other definitions for area that I've seen before include "Region, zone" , "part of town" , "'Tract, region (4)'" , "Space for a specific purpose, say picnic" , "Surface measurement" .)