A portion of haggis always lacks flavour at first (4)
I believe the answer is:
half
'a portion' is the definition.
(I have seen 'A portion' mean 'half' so perhaps 'portion' could also mean 'half')
'haggis always lacks flavour at first' is the wordplay.
'at first' indicates taking the first letters.
The first letters of 'haggis always lacks flavour' is 'HALF'.
'of' acts as a link.
(Other definitions for half that I've seen before include "Moiety" , "Fifty per cent" , "Measure of beer" , "one over a couple" , "Short distance - steal!" .)