Design outfit periodically welcomed by both genders (5)
I believe the answer is:
motif
'design' is the definition.
(I know that motif is a type of design)
'outfit periodically welcomed by both genders' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'welcomed by' is an insertion indicator (letters welcomed inside).
'both genders' becomes 'mf' (I am not sure about this - if you are sure you should believe this answer much more).
The alternate letters of 'outfit' are 'oti'.
'oti' placed into 'mf' is 'MOTIF'.
Can you help me to learn more?
(Other definitions for motif that I've seen before include "Distinctive idea or theme" , "recurring idea" , "Logo -- pattern" , "Single design - theme" , "Theme or melody" .)