Bridge declaration — going first is one for the old grey cells (9)

'one for the old grey cells' is the definition.
(original name for the crossword)

'bridge declaration going first' is the wordplay.
'bridge' becomes 'cross' (I've seen this in other clues).
'declaration' becomes 'word' (a word can be spoken or declared).
'going first' means one lot of letters go next to another.
'cross' after 'word' is 'WORDCROSS'.

'is' acts as a link.