Fierce wife is lady every so often (4)
I believe the answer is:
wild
'fierce' is the definition.
(similar in meaning)
'wife is lady every so often' is the wordplay.
'wife' becomes 'w' (abbreviation).
'every so often' indicates alternate letters.
'is'+'lady'='islady'
The alternating letters of 'islady' are 'ild'.
'w'+'ild'='WILD'
(Other definitions for wild that I've seen before include "Not tamed" , "Born Free?" , "Tempestuous" , "Feral - crazy" , "Impulsive" .)