Fruit twice yearly gaining weight (6)
I believe the answer is:
pawpaw
'fruit' is the definition.
(pawpaw is a kind of fruit)
'twice yearly gaining weight' is the wordplay.
'twice' shows that letters should be duplicated.
'yearly' becomes 'pa' (abbreviation for per annum).
'gaining' is a charade indicator (letters next to each other).
'weight' becomes 'w' (short for weight).
'pa'+'w'='paw'
'paw' doubled is 'PAWPAW'.
(Other definitions for pawpaw that I've seen before include "Fruit" , "Papaya" , "Custard apple tree" , "foul once" .)