Hand tool to cross nerd not all there (7)
I believe the answer is:
spanner
'hand tool' is the definition.
(I know that spanner is a type of hand tool)
'cross nerd not all there' is the wordplay.
'cross' becomes 'span' (span can mean to cross or bridge).
'not all there' means to remove the last letter.
'nerd' with its final letter taken off is 'ner'.
'span'+'ner'='SPANNER'
'to' acts as a link.
(Other definitions for spanner that I've seen before include "In the works, this tool is an obstacle" , "that might cause difficulty in the works" , "In the works, this tool is a headache" , "Type of wrench" , "Hand tool for nuts" .)