Poor Rebecca's beau keeps uniform for dressing (8,5)
I believe the answer is:
barbecue sauce
'dressing' is the definition.
(I've seen this before)
'poor rebecca's beau keeps uniform' is the wordplay.
'poor' indicates anagramming the letters.
'keeps' indicates putting letters inside.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'rebeccas'+'beau'='rebeccasbeau'
'rebeccasbeau' with letters rearranged gives 'barbecesauce'.
'barbecesauce' placed around 'u' is 'BARBECUE SAUCE'.
'for' is the link.
(Another definition for barbecue sauce that I've seen is " Dressing".)