Small hint first -- you'll need these to fill the grid! (5)
I believe the answer is:
clues
'you'll need these to fill the grid' is the definition.
(solvers need clues to fill a crossword grid)
'small hint first' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'hint' becomes 'clue' (synonyms).
'first' means one lot of letters go next to another.
's' after 'clue' is 'CLUES'.
(Other definitions for clues that I've seen before include "Here, they're across and down" , "Crossword aids" , "Hints (that you're looking at?)" , "Indications" .)