Be last to contact certain soldiers in pub (5,2,3,4)
I believe the answer is:
bring up the rear
'be last' is the definition.
(I've seen this before)
'contact certain soldiers in pub' is the wordplay.
'contact' becomes 'ringup' (to ring up is to telephone).
'certain' becomes 'the' (definite article - refers to a certain object).
'soldiers' becomes 're' (Royal Engineers).
'in' means one lot of letters goes inside another.
'pub' becomes 'bar' (I've seen this before).
'ringup'+'the'+'re'='ringupthere'
'ringupthere' placed within 'bar' is 'BRING UP THE REAR'.
'to' acts as a link.
(Other definitions for bring up the rear that I've seen before include "come last" , "Trailers do" , "Come at the end" .)