Polite Dicky carries cross in heroic deed (7)
I believe the answer is:
exploit
'heroic deed' is the definition.
(I've seen this before)
'polite dicky carries cross' is the wordplay.
'dicky' is an anagram indicator (dicky can mean ill or non-functioning).
'carries' is an insertion indicator (inserted letters carried inside).
'cross' becomes 'x' (X is cross-shaped).
'polite' anagrammed gives 'eploit'.
'eploit' going around 'x' is 'EXPLOIT'.
'in' acts as a link.
(Other definitions for exploit that I've seen before include "Valiant deed; take advantage of" , "Make use of - great deed" , "Heroic feat" , "Feat, accomplishment" , "Make use of - achievement" .)