Engineers welcomed by the cardinal (5)
I believe the answer is:
three
'cardinal' is the definition.
('three' can be similar in meaning to 'cardinal')
'engineers welcomed by the' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'welcomed by' indicates putting letters inside (I've seen this in other clues).
're' placed within 'the' is 'THREE'.
(Other definitions for three that I've seen before include "There (anag.)" , "Number of feet in a yard" , "So many 12s today" , "Amount in a trio" , "Number of blind mice" .)