Hold up, crossworders, the first of these is tough (6)
I believe the answer is:
robust
'these is tough' is the definition.
(robust means tough or strong)
'hold up crossworders the first' is the wordplay.
'hold up' becomes 'rob' (as in holding up a bank).
'crossworders' becomes 'us' (I can't justify this - if you can you should believe this answer much more).
'the first' becomes 't' (ust letter of 'the').
'rob'+'us'+'t'='ROBUST'
'of' is the link.
Can you help me to learn more?
(Other definitions for robust that I've seen before include "Strong and sturdy" , "Having a strong constitution" , "Physically strong" , "Vigorous - healthy" , "Sturdily built" .)