Solidarity's leader embraced by distressed seamen all at once (2,5)
I believe the answer is:
en masse
'all at once' is the definition.
(thesaurus)
'solidarity's leader embraced by distressed seamen' is the wordplay.
'leader' suggests taking the first letters.
'embraced by' is an insertion indicator.
'distressed' indicates anagramming the letters.
The first letter of 'solidaritys' is 's'.
'seamen' is an anagram of 'enmase'.
's' going into 'enmase' is 'EN MASSE'.
(Other definitions for en masse that I've seen before include "as a bunch" , "All together (Fr.)" , "All together - in France (2.5)" , "in concert" , "All together as a group" .)